Circadian dysregulation and Alzheimer’s disease: A comprehensive review

Abstract

Alzheimer’s disease (AD), the foremost variant of dementia, has been associated with a menagerie of risk factors, many of which are considered to be modifiable. Among these modifiable risk factors is circadian rhythm, the chronobiological system that regulates sleep‐wake cycles, food consumption timing, hydration timing, and immune responses amongst many other necessary physiological processes. Circadian rhythm at the level of the suprachiasmatic nucleus (SCN), is tightly regulated in the human body by a host of biomolecular substances, principally the hormones melatonin, cortisol, and serotonin. In addition, photic information projected along afferent pathways to the SCN and peripheral oscillators regulates the synthesis of these hormones and mediates the manner in which they act on the SCN and its substructures. Dysregulation of this cycle, whether induced by environmental changes involving irregular exposure to light, or through endogenous pathology, will have a negative impact on immune system optimization and will heighten the deposition of Aβ and the hyperphosphorylation of the tau protein. Given these correlations, it appears that there is a physiologic association between circadian rhythm dysregulation and AD. This review will explore the physiology of circadian dysregulation in the AD brain, and will propose a basic model for its role in AD‐typical pathology, derived from the literature compiled and referenced throughout.

1 Background

The relationship between circadian rhythm disruption and Alzheimer’s disease has been explored over the century since Alzheimer’s disease (AD) was discovered, however it is only in recent years that circadian disruptions have been recognized as a potential propagator of AD onset and progression [1]. Recent studies involving diverse subject groups from nursing home residents to individuals who live in regions with irregular light‐dark cycles have shown that circadian disruption may be a modifiable risk factor for AD.

AD is the foremost neurodegenerative disease principally characterized by the formation of senile plauqes comprised of amyloid‐β (Aβ) peptide accumulation and hyperphosphorylation of the tau protein which forms neurofibrillary tangles (NFTs) [2]. Getting a better understanding of this disease’s effects on the sleep patterns of its victims is necessary for developing better therapeutic approaches and allowing for earlier diagnoses.

Circadian rhythms exist at the cellular level, as has been shown through experimental studies involving microorganisms, and at the human level the circadian clock, located within the suprachiasmatic nucleus (SCN) of the hypothalamus, regulates physiological processes in an oscillatory fashion. It is important to note that cells, referred to in this paper as circadian‐regulating cells are those neurons which are associated with the SCN or as well as cells associated with peripheral oscillators, and are shown to have an effect on regulating, or are regulated by the circadian rhythm. This includes cells who express binding sites for biomolecules known to mediate regulatory circadian processes, as well as cells that synthesize those biomolecules. In addition, cells and other biological structures discussed later in this review whose functionality is altered in response to the ambient conditions of the host’s environment, are also considered circadian cells.

While the purpose of the circadian clock is to sync physiological function to a 24‐hour temporal cycle, this regulatory process is in large part dependent on the ambient conditions of one’s environment [3]. Because light exposure seldom remains constant on a day to day basis for any individual, the circadian clock tends to run slightly over the standard 24‐hour period, but resets itself in accordance with light/dark cycles. When light exposure levels are optimized, circadian rhythms will function normally, and the circadian clock will be able to reset itself in accordance with proper light/dark cyclical exposure, provided that there are no endogenous irregularities which will preclude the circadian clock from normal function [4]. Dysregulation of the circadian rhythm will occur primarily in individuals who are irregularly exposed to light including shift workers, nursing home residents, inmates, and individuals who live in regions with irregular light/dark cycles [4, 5].

Among the physiological functions regulated by circadian rhythm are sleep/wakefulness cycles, cellular activity, gut microbiome health, body temperature, and immunal optimization [3, 6]. The circadian clock plays these regulatory roles by stimulating the meticulous secretion of specific neurotransmitters and synthesis of hormones in response to light exposure in its host organism’s environment [6]. The primary hormones involved in the regulation of circadian effects on physiological activity are melatonin, which promotes sleep, and cortisol, which promotes wakefulness; as well as serotonin, which has been shown to act as a mediator of circadian rhythmicity [4]. However a number of other circadian rhythm regulating hormones, neurotransmitters, protein, and neuromodulators secrete when triggered by environmental conditions as well, including vasopressin, acetylcholine, glutamate, etc.

Melatonin has been shown to act on many critical cerebral organs, including the SCN itself. Specifically, melatonin levels have been shown to suppress the release of cortisol, and thus attenuates the SCN signaling which would otherwise induce cortisol secretion [7]. Moreover, melatonin abundance has proven to be directly correlated to the duration of sleep, with the deepest sleep periods coming about 2 h after the initial onset of melatonin secretion [8]. Because sleep consists of two primary phases, rapid eye movement (REM) and non‐REM sleep, both of which are critical to the restorative function of sleep, disruption of the timing and duration of these cycles can impair the rejuvenation that sleep would otherwise offer [9]. Importantly, there is a distinct dissimilarity between endogenous melatonin, which is the type that acts on the SCN and inhibits cortisol secretion, and exogenous melatonin, which is frequently administered as a sleep promoter [10]. This is a nuance that will be explored more deeply in this review.

Because of the regulatory role that both melatonin and cortisol play in the elicitation of sleep and wakefulness, respectively, it is imperative that they be characterized as the primary drivers behind circadian function as a result of exogenous activity, such as light exposure [11–13]. In addition, serotonin should be considered a mediatory hormone that promotes the optimization of circadian phase timing and duration. However, the other hormones and neurotransmitters that will be touched on later in this review, are necessary and important for circadian function in mammals and the regulation of various tangential rhythms and tissue functions throughout the body, which fall under the umbrella of circadian regulation. Furthermore, none of these hormones is more critical to the optimization of circadian rhythm than environmental light exposure itself, as it is a direct modifier of melatonin and cortisol abundance [14]. The role of light exposure as well as the interactions between melatonin, cortisol, serotonin, and all other hormones involved in circadian rhythm will be explored in detail within this review.

While the study of circadian rhythm is in its infancy, relative to other physiological fields of study, the basic tenets of its physiological makeup and function are fairly well understood, and no one would argue that its optimization is necessary for the prevention of downstream adverse effects. But, its relationship with neurodegeneration has not been studied extensively, and is certainly not well understood. This is due in part to the difficulty of studying sleeping subjects. While the technology which enables researchers to obtain physiological data from an individual in a sleep state continues to advance, there are questions raised pertaining to the validity and breadth of this data [15]. A common gripe with physiological data collected during sleep is the fact that this data collection, by its own nature, presents the study participant with an unnatural and, in all likelihood, uncomfortable sleep state, that is a departure from their standard sleep state [16]. For example, if one were to take a simple measurement, such as skin conductance temperature collected through an electric node strapped to the finger of a participant and hooked up on the backend to a monitor for data recording, this slight difference between the study condition and their normal sleep state may lead to increased arousal or shorter/nonexistent periods of deep REM sleep, which could cause the data collected to be misleading. For this reason, there may be a disparity between the data collected during a sleep study, and the true physiological state of a participant when they are unimpeded by scientific machinery.

Furthermore, while a better understanding of the physiological purpose of sleep is being garnered each year, many of the findings related to sleep’s purpose for the human mind and body are either inconclusive or entirely theoretical [17]. As of now (2022) the scientific community only has a cursory understanding of the many benefits attributed to a good night’s rest. It seems apparent that memory consolidation and the pruning of excess neurons (elimination of cortical matter that is, in effect, “dead weight”) are drastically upregulated during sleep, and the immunal and bodily recovery benefits of sleep have been well documented [18, 19]. However, given the observed restorative effect that sufficient sleep can have on the human body and mind, it is entirely likely that these findings only scratch the surface of the benefits accrued with optimized sleep patterns over time. Many of these findings will remain out of reach until they are unbound by technological or methodological advancement.

In addition, the fact that the study of Alzheimer’s disease itself has yet to reveal a causative mechanism is yet another barrier to understanding the relationship between circadian clock function and AD‐typical neurodegeneration. All data at this point that may speak to a correlation between these two areas of interest has to be contextualized in accordance with symptoms and physiological processes related to AD, rather than any firm physiological mechanism that drive the onset and the progression of the disease itself [20]. Therefore, the correlations between the circadian clock and an AD consequence or symptom, will be proportional to the degree of relatedness between that consequence or symptom and AD itself. As a hypothetical example, if it was observed that there was a correlation between circadian clock dysregulation and the deposition of Aβ in the brain, we could conclude that this is a correlation firmly related to AD, as the relationship between Aβ and AD is firmly established and there is a direct correlation between AD severity and the level of Aβ deposition. Therefore, we could infer that an observed correlation between upregulated Aβ deposition and circadian dysregulation, may speak to a larger correlation between AD and circadian dysfunction. On the other hand, if we observed that there was a correlation between dysregulated circadian rhythm and increased expression of the TREM1 gene, this finding is less indicative of a firm correlation between AD and circadian irregularities, as heightened TREM1 expression in the AD brain has been observed, but is poorly understood and understudied.

Despite these limitations, there is sufficient literature to review in relation to the correlation between Alzheimer’s disease and circadian rhythms. Numerous studies have been collected in various populations from nursing home residents, to shift workers and individuals who live in regions with irregular light/dark cycles.

Giggins et al. used activity monitors to assess sleep/wake cycling and activity timing in a sample of ten nursing home residents [21]. The team cycled lighting in the respective environments of the residents to mimic normal daylight, and found that there was a mix between positive and adverse physiological and behavioral responses. Those that responded positively to the light‐cycling intervention showed improvements in the regularization of sleep, and an improvement in overall mood based on subjective mood rating assessments administered throughout the study. There was however a subset of the participants who showed no change in sleep/wakefulness regularity or mood [21].

Perhaps the most pertinent evidence for an association between AD and altered circadian patterns is the fact that in the preclinical stages of AD, alterations in circadian rhythmicity have been observed consistently, which many studies have shown to be an effective diagnostic marker of AD [1, 22]. Targa et al. studied circadian clock patterns in 100 patients with mild to moderate AD using actigraphy analyses over a two week period to show that there was an increased fragmentation of circadian rhythm in patients who expressed high levels of Aβ deposition and tau hyperphosphorylation, which was determined by collecting cerebrospinal fluid samples from participants. Follow up analyses with the participants one year after the study revealed that cognitive decline had worsened, both physiologically and behaviorally over time, and that circadian rhythm was dysregulated to an even greater extent [22].

Not only do these findings speak to the correlation between AD and the circadian clock and its potential use as a diagnostic marker, but they also solidify circadian rhythm dysregulation as a risk factor for AD. While interventions have been attempted in this brain area, it remains to be seen whether or not circadian dysfunction is a modifiable risk factor. This is a topic that will be explored in great detail throughout this review. As an exogenous risk factor, meaning that its dysfunction can be regulated by external modifiers (in this case light exposure or hormonal therapy), the circadian clock could prove to be a fertile area for intervention based therapeutics that may aid in the ongoing efforts to effectively alleviate AD symptoms in the human brain. However, a better understanding of the mechanisms that drive it, both in the AD and healthy brain, is necessary to develop efficient mechanisms by which it can be used to both diagnose AD, and intervene in the pathophysiological processes of the disease.

2 Alzheimer’s physiology

To fully understand the relationship between circadian rhythm and AD, it is imperative to understand the known physiopathology that underlies Alzheimer’s disease. The neuro‐degenerative disease, which affects one in nine Americans and is the sixth leading cause of death, is primarily characterized by two physiological symptoms [23].

First, there is an accumulation and oligomerization of Aβ, which form senile plaques resting in and around synaptic junctions, thus blocking neuronal transmission and resulting in a progressive loss of neuronal plasticity [24]. Aβ is derived from the amyloid precursor protein (APP), which is sequentially cleaved along one of two pathways: the amyloidogenic or non‐amyloidogenic pathway. The former, involving the proteolytic cleavage of the protein by β‐secretase (BACE1) and γ‐secretase, is the pathway of interest, as it is the cleavage pathway that yields the Aβ peptide and APP intracellular domain (AICD) as its products. The non‐amyloidogenic pathway on the other hand, which involves cleavage by the proteases α‐secretase and γ‐secretase, yields C‐terminal fragment α (CTFα, regarded as a neurotrophic factor), and extracellular APP. Cleavage along the non‐amyloidogenic pathway is downregulated in the AD brain, while cleavage along the amyloidogenic pathway is upregulated [25]. As a result of this spike in amyloidogenic cleavage, the Aβ peptide is produced in a progressively increasing quantity, which leads to its heightened accumulation and increased senile plaque formation as the disease progresses [26].

Second, the hyperphosphorylation of the tau protein in the AD brain leads to the formation of neurofibrillary tangles (NFTs), which are aggregations of the hyperphosphorylated microtubule associated protein (MAP) variant [27]. Given that the tau protein is MAP, researchers assumed that upon its discovery in 1975, it behaved like other MAP proteins better known at that time [28]. They were partly correct, as it was discovered that the tau protein is similar in both structure and function to the MAP2 protein, which is found primarily in dendrites and works to stabilize microtubules through conserved C‐terminal microtubule binding domain and N‐terminal domains which project from the proteins [29]. The tau protein has been discovered to serve a very similar function in the healthy brain, however much more unexpected were the discoveries of its role as a pathological symptom of AD. In the AD brain, the tau protein becomes hyperphosphorylated, meaning that the protein, which contains multiple phosphorylation sites used for the regulation of various biochemical processes including the regulation of signaling cascades and genetic expression, is fully saturated with phosphate molecules [30]. This process is thought to be triggered by either the initial deposition and accumulation of Aβ and/or a dysregulated brain glucose metabolism, both of which are known physiological symptoms of AD [31]. The latter of these two proposed causal mechanisms for the hyperphosphorylation of tau is supported by the findings of An et al. (2018), who showed that in autopsied victims of AD, there is a marked increase in brain glucose and downregulation in the glycolytic flux regulating protein GLUT3, which is correlated with increases in tau hyperphosphorylation [32]. The tau protein is responsible for the stable formation of microtubules, which are cytoskeletal components of eukaryotic cells, however when it is hyperphosphorylated, the protein misfolds to form paired helical filaments (PHFs) which subsequently become entangled, thus forming NFTs [33]. Although the exact reason has not yet been elucidated, there is a correlation between the increased abundance of NFTs, and an increased rate of apoptosis [33, 34]. This is thought to be because, like the senile plaques of Aβ, NFTs form blockages at synaptic junctions, which lead to a decrease in the rate of axonal action potential propagation and decrease in dendritic reception and branching, and thus a decrease neuronal plasticity [34, 35].

Both of these AD‐typical consequences are universal features of AD, however whether or not they are the direct cause of the disease or merely a consequence of a different underlying causal mechanism is a hotly contested topic. Given the fact that Aβ and tau accumulation and blockages are universally present features of AD, it is reasonable to assume that the two are directly causal, however this argument does not have purchase for several reasons. Chief among them being that experimental therapeutics designed to target and reduce these accumulations thus far have found success in doing so, and some, such as the recently Food and Drug Administration (US)‐approved Aducanumab, have even shown some efficacy in slowing the progression of the disease for a time [36]. However none of these novel therapeutics have been successful in halting the progression of the disease, even when they were successful in clearing Aβ and/or NFTs from the AD‐burdened brain [37]. To this point, a highly influential study in 2000 by Mucke et al., found that targeted clearance of Aβ plaques in transgenically modified murine brains had no effect in slowing the disease progression or deposition of Aβ, in spite of the fact that all senile Aβ plaques were cleared from the brains of the mice in question [38].

In the study, Mucke and his team encoded four minigenes as wild‐type human amyloid precursor protein (hAPP) or as hAPP that carried mutations that altered the production of amyloidogenic Aβ peptides, which are the constituent ingredients in amyloid plaques. The latter of these encoding options was designed to mimic FAD (familial Alzheimer’s disease). It was observed that increased cerebral Aβ levels gave rise to amyloid plaques that increased in density proportional to the severity of the illness, in those transgenically modified mice who had been encoded with the FAD hAPP mutations. But, the mice who had been encoded with the wild‐type hAPP did not see an increase in amyloid plaque levels. These wild‐type mice, just like the FAD hAPP transgenically modified mice, saw massive decreases in SYN‐IR presynaptic terminals. Furthermore, Mucke found that, across mice from different transgenic lines, the density of SYN‐IR presynaptic terminals is inversely correlated to the levels of Aβ, but not with hAPP levels or plaque volume. The conclusion from this study was that Aβ is synaptotoxic, even without plaques, and high levels of Aβ are not enough to elicit the formation of plaques in mice who express wild‐type hAPP. In its purest interpretation, this study plays an important role in parsing Aβ and its neurotoxicity from the amyloid plaques which, while correlated, may not be causal in any specific way. Mucke and colleagues showed that, under the assumption that SYN‐IR presynaptic terminals and the prevalence with which they exist are a valid indicator of the AD’s severity, that Aβ is an independent operator, at least as it exists in its wild‐type variation [38].

The findings of this study, as well as the failure of therapeutic treatments aimed at reducing plaque load and tau burden, provide evidence for the fact that Aβ accumulation and tau hyperphosphorylation, while relevant to the progression of AD, are not causal although even more study is needed to substantiate this assumption. But, it seems likely that AD is a multifactorial disease triggered by a series of underlying physiological mechanisms. The study of these potential mechanisms of AD onset and progression are inconclusive, and the research surrounding many processes that have thus far been proposed and tested, is either still in its infancy or has yielded no result in stopping the progression of AD [39].

Among these processes is the dysregulation of genes that have been proven to be associated with AD, most notably apolipoprotein epsilon 4 (ApoE4), whose expression is one of the foremost predictors of AD onset [40]. Nearly 60% of individuals who express two copies of the ApoE4 gene are burdened with AD later in life [41]. It is thought by many that the expression of the ApoE4 gene is related to impaired clearance of Aβ across the blood brain barrier. Another strong genetic predictor of AD is the PSEN gene, and its associated protein presenilin 1 (PS1) has been shown to be highly upregulated in the brains of AD patients [42]. There is a host of other genes that have been correlated to the function of APP, deposition of Aβ, and hyperphosphorylation of tau including, TREM, FE65, SHARPIN, LRP1, CCR5, MAP2K7, and MAP3K7, among others, however their role is not well understood [42].

The dysregulation of nuclear factor κB (NF‐κB), a gene associated with neuroinflammation, is a firmly established physiological symptom of AD pathology [43]. When NF‐κB binds to cellular DNA, signaling cascades are induced that are known to drive neuroinflammation. Primarily, the nitric oxide synthase (iNOS) and cyclooxygenase (Cox) genes will be activated through the NF‐κB to DNA bond, and this activation will lead to the production of proinflammatory cytokines including interleukin (IL)‐1β, IL‐6, and tumor necrosis factor (TNF)‐ɑ, among others [44]. This process will exacerbate neuroinflammation, which is known to be instrumental in the onset and progressive exacerbation of physiological and behavioral AD symptoms [45].

Oxidative stress is also a known driver of AD pathology, and radical oxygen species (ROS) are free radicals thought to drive AD progression and are often used as a biomarker for the diagnosis of AD pathology [46]. As free radicals, ROS overabundance can cause damage to important lipids, DNA, and proteins necessary for optimal cognitive function. Neuronal lesions induced by the hallmarks of AD pathology, Aβ plaques and NFTs, will heighten the activity of microglia, and this increased immune response will make the microglia and neurons themselves more vulnerable to oxidative damage due to their increase in metabolic demand, lower rate of cellular regeneration in the diseased brain, and diminished abundance of antioxidants as a result of the disease pathology [47]. Furthermore, the clearance rate of free radical toxins from the brain will be diminished as a result of the decreased abundance of free radical scavengers, such as melatonin in the diseased brain [48]. This will allow free radicals such as ROS to linger and inflict more damage for a longer period than would otherwise be permitted [49]. This will contribute to direct cellular damage and will sustain neuroinflammation, thus exacerbating AD symptoms over time.

Other processes such as the expression of intra‐CNS myelin regeneration inhibitors, atypical upregulations in apoptotic ligand binding, prionism, and neuroinflammatory processes involving micro‐ and astrogliosis have been proposed and may offer fertile ground for the development of AD‐halting therapeutics, however their study is still in its infancy and thus their potential efficacy as targets for AD treatment is not particularly relevant to the purposes of this review [50–52].

3 Circadian clock physiology

As mentioned, the circadian clock is anatomically defined as the suprachiasmatic nucleus (SCN), a structure housed in the anterior portion of the hypothalamus [53]. The SCN is comprised of about 10000 neurons and is positioned directly superior to the optic chiasm [53]. As noted by Ma and Horrison (2021), the anatomical structure of the SCN can be subdivided into two different classes of subregions; the core subregions and the shell subregions [54]. The core subregions produce the vasoactive intestinal peptide (VIP) and the gastrin‐releasing peptide (GRP), while the shell subregions are made up of arginine vasopressin (AVP) expressing cells [54].

These subregions have been shown to express their respective components differently in correlation with both exogenous and endogenous cues [55]. In the core subregions, the abundance of VIP is highest during sleep periods in response to darkness, while GRP peaks during wakefulness in response to light exposure. AVP regions on the other hand are cued endogenously, although the specific mechanisms which lead to heightened electrical activity in the AVP expressing neurons are not well known [56]. It has been observed that AVP expression peaks just before sleep in murine models, and there is a correlation between heightened AVP expression and increased feelings of hunger and thirst, indicating that AVP is largely responsible for the regulation of eating and drinking patterns in mice, and possibly other mammals [57]. AVP neurons project to the paraventricular nucleus, which is a nuclei cluster in the hypothalamus that is known to regulate essential processes such as cellular metabolism, stress and arousal, and growth to name a few [56]. There is a possible correlation between the regulatory activity in the AVP‐expressing shell subregion of the SCN and the activity of the paraventricular nucleus, which may mean that degeneration or dysregulation in one of these structures, can have downstream effects that may impair the function of the other [54].

Research in hamsters has revealed that the health of the afferent and efferent neural pathways of the SCN are critical for the optimization of the circadian rhythm [58]. The afferent projections of the SCN are responsible for the reception and processing of photic information, and these projections include the retinohypothalamic tract (RHT) and the geniculohypothalamic tract (GHT), which stems from the intergeniculate leaflet (IGL) [54, 58]. Non‐photic afferent projections on the other hand, process endogenous cues for circadian regulation, and these include the projections from the median raphe nucleus and the pedunculopontine, parabigeminal, and laterodorsal tegmental nuclei [54, 58]. All of these tracts project to both the core and shell subregions of the SCN, although they are more sparsely scattered throughout the shell, and more densely populated in the core of the SCN [54].

The RHT is responsible for the reception of photic stimuli, as it receives inputs from photosensitive ganglion cells within the eye, specifically in the retina [59]. Within these photoreceptive ganglion are a peptide called pituitary adenylate cyclase‐activating polypeptide (PACAP), which is involved in the photic information transmission and potentiation of glutamate along the RHT. This will in turn activate the VIP regions in the core of the SCN, regulating its expression in accordance with the amount of light being perceived. The RHT receives inputs from photosensitive ganglion cells within the eye, specifically in the retina [54, 59].

The GHT is also a key mediator of photic inputs from the retina to the SCN, however it can also be cued endogenously by muscle contraction and cognitive activity [60]. GHT activity is primarily mediated by neuropeptide Y (NPY), enkephalin (ENK), and GABA, with NPY abundance having the most influence on the regulation of neurons within the SCN that act as rhythmic pacemakers [58]. As a result, dysregulation in NPY mediation and abundance has been shown to induce phase‐shifts in biological rhythms [53, 61]. GABA dysregulation has been shown to facilitate the untimely depolarization of SCN pacemaker neurons, which leads to excitatory activity during the sleep phase of the circadian cycle [62]. However, the function of GABA in both the healthy and dysregulated circadian rhythm is poorly understood, and in need of more extensive research.

The raphe nuclei are instrumental in the regulation of serotonin secretion, which has the ability to increase or decrease the expression of VIP neurons in the core regions of the SCN [54]. Serotonin has been shown to potentiate the release of glutamate during waking hours, and inhibit the release of glutamate during sleep [63]. Optimal abundance of glutamate is responsible for the regulation of the aforementioned pacemaker neurons in the SCN, and is therefore necessary for circadian optimization [62]. Alterations to the median raphe nuclei can lead to imbalance in this potentiation‐inhibition regulatory role of serotonin, and can lead to irregularities in the sleep–wake cycle [64]. Deficient or abnormally heightened serotonin abundances can either downregulate or upregulate the firing rate and expression of pacemaker neurons through the mediation of glutamate, and his can lead to undue excitability during sleep hours or fatigue during waking hours, depending on the relative abundance of serotonin [63]. This being the case, the median raphe nucleus can be regarded as a key regulatory structure for the optimization of circadian rhythm.

The tegmental nuclei aforementioned (pedunculopontine, parabigeminal, and laterodorsal tegmental nuclei) employ cholinergic neuro‐transmitters to regulate the function of the SCN, although the specifics of their involvement have remained elusive to researchers thus far [65].

As for the efferent projections of the SCN, monosynaptic and polysynaptic efferent neurons appear to serve different functions, although their specific roles are in need of more study [54]. Monosynaptic efferent neurons appear to be involved primarily in intra‐thalamic regulatory processes, as these efferent projections tend to terminate in various nuclei within the hypothalamus and thalamus [66]. Different monosynaptic projections appear to play various roles, depending on the structure in which they terminate. For example, monosynaptic projections which terminate in the preoptic nucleus of the hypothalamus, have been shown to play a key role in the regulation of sleep and wakefulness timing. This is likely due to the fact that the clock‐like signaling of the pacemaker neurons within the SCN are acting via these efferent projections to induce desired activities in the preoptic nucleus, which is responsible for thermoregulation among other things [58, 67]. Therefore, close to the sleep phase in the sleep–wake circadian cycle, the SCN could be transmitting information along these monosynaptic efferent projections to the preoptic nucleus telling it to heat the body up for sleep in an otherwise cold environment [68]. Once again, more study is needed to identify the specific efferent signaling cascades that are taking place in the context of circadian regulation and monosynaptic projection from the SCN, however the same basic signaling process discussed in the example above, can likely be assumed of all monosynaptic efferent projections from the SCN to get other nuclei within the thalamus and hypothalamus to fulfill their necessary role for circadian regulation at a given time.

Polysynaptic neurons play a slightly more extra‐thalamic role, in that they terminate further from the SCN in different parts of the brain, most notably the pineal gland, which is responsible for the production and secretion of melatonin [60]. During the sleep phase of the circadian sleep–wake cycle, polysynaptic efferent projections from the SCN release norepinephrine, which is subsequently involved in the activation of β1 and β2 pinealocyte adrenergic receptors [60]. This norepinephrine‐pinealocyte activation process will elicit the production of melatonin, one of the key regulating hormones involved in circadian regulation [60].

Throughout the year, as days lengthen and shorten, different abundances of the hormones relevant to circadian rhythm are required to optimize circadian function [69, 70]. Relative to the length of day and amount of light and darkness throughout that particular day, the master oscillator will relay information based on the afferent photic input that it has received, and utilize SCN pacemaker neurons to transmit instructions by way of the efferent projections mentioned above [58]. From the information projected by these efferent signaling pathways, different signaling cascades will be induced, and hormonal signaling by way of the major circadian‐related messengers will trigger functional alterations in the circadian cells present within the tissue of various organs throughout the body including the liver, kidneys, lungs, and heart to name a few [58, 71]. The true nature of the signaling cascades that induce physiological changes as a result of photic and nonphotic regulation of the SCN and its closely associated structures, is unknown as of now [72]. However, what is known, is that these physiological alterations aimed at optimizing the circadian rhythm are mediated by several key hormones, namely melatonin, cortisol, and serotonin [73]. The details of how each of these major hormones serves to mediate circadian signaling and induce physiological alterations in accordance with light and darkness exposure, as well as endogenous regulators, will be explored individually in the following sections [74].

It is also important to note the genetic mechanisms that serve to regulate the function of the master oscillator and peripheral oscillators. A series of genes are closely associated with circadian regulation, including BMAL1/2, CLOCK, PER1/2/3, and CRY1/2 [75]. These genes are expressed intracellularly and they exert great influence on the transcription and translation processes involving their associated proteins that are thought to be involved in circadian processes [75, 76]. Expression of these genes intracellularly is critical to the regulation of the aforementioned signaling pathways involving melatonin, serotonin, and cortisol, as well as the SCN and its afferent/efferent projections. The phosphorylation of the proteins derived from these clock regulating genes is known to be an important mechanism for the control of rhythmic oscillation, as this process of phosphorylation will lead to protein degradation that regulates the abundances of these proteins, thereby maintaining the equilibrium necessary for circadian optimization [77]. Furthermore, while the expression of these genes on cells in various tissues of circadian rhythm‐related organs throughout the body is indeed important, it is also important to note that circadian regulation can be exerted externally, as cells with no nuclei also are subject to SCN regulation [62]. While the specifics of these regulatory processes on cells are not fully understood, it is likely that cells without nuclei‐expressing clock‐regulating genes, are more subject to influence by the key hormones released as a result of SCN‐induced signaling cascades, while cells that do possess nuclei‐expressing clock‐regulating genes rely more on the genes themselves for regulation and rhythmic alteration in accordance with exogenous cues [62]. Nonetheless, the genetic mechanisms involved in circadian regulation are undoubtedly necessary for circadian optimization on a cell to cell level, and are, in all likelihood, complementary to the signaling cascades and mediatory hormones induced by the SCN itself [78].

4 Melatonin

The circadian rhythm is tightly regulated in humans, and the key component of this highly regulated cycle is the hormone melatonin, an indolic hormone which itself is a derivative of serotonin as it is produced via the tryptophanserotonin biosynthetic pathway in the pineal gland [79]. In essence, there is a substantial rise in endogenous melatonin levels as darkness begins, and an equivalent decrease upon waking and/or light exposure [80]. Melatonin is photicly regulated meaning, that when a human is exposed to light, endogenous melatonin levels will be reduced dramatically [80, 81]. This light‐dependent melatonin inhibition occurs at several different levels. It has been firmly established that light reduces the abundance of melatonin circulating in the blood [81]. It has also been observed that there is a reduction in melatonin in blood plasma, which is the result of a reduction in the synthetic enzymes associated with melatonin in the pineal gland when exposed to light [81, 82]. It has been shown that there is a marked reduction in the expression of the melatonin synthetic enzyme N‐acetyltransferase in the human pineal gland during daylight hours, giving credence to this idea [83].

As mentioned, endogenous melatonin, the more pertinent melatonin variant in the study of circadian rhythm, is derived from the tryptophan and serotonin biosynthetic pathways in the pinealocyte [84]. During this process, tryptophan enters the cell and interacts with two enzymes, tryptophan‐5‐hydroxylase and 5‐hydroxytryptophan decarboxylase, in a process that yields serotonin. This serotonin then goes through a process of acetylation by arylalkylamine‐N‐acetyltransferase (AA‐NAT), and then methylation by acetylserotonin‐O‐methyltransferase (ASMT). From this two step process, the acetylated and methylated serotonin yields melatonin [85].

In the context of circadian regulation, endogenous melatonin can be used for regulatory signaling cascades involved in circadian rhythm [86]. The signal transduction process for endogenous melatonin begins in the membrane of the pinealocyte [81]. Here, cervical ganglion cells possess adrenergic receptor sites, β1 and α1, receiving adrenergic neurons [87]. The β1 and α1 receptors are stimulated by norepinephrine released via the fibers associated with the efferent projections from the SCN during a sleep state [87]. The norepinephrine potentiation will increase the production of calcium ions via the α1 receptor, while in the β1 receptor, it activates adenylate cyclase which will increase cyclic adenosine monophosphate (cAMP) in the cytoplasm. In turn, cAMP depends on protein kinase A (PKA), which will increase the production of the enzyme N‐acteyltransferase [81, 88].

While this is the master process by which melatonin is derived and secreted, it is worth noting that many different extra‐pineal tissue systems throughout the body also possess biosynthetic mechanisms that produce melatonin locally, albeit in smaller amounts than what is produced and secreted by the pineal gland. Among these local cellular sites of melatonin synthesis and secretion are lymphocytes, bone marrow, the gastrointestinal tract, epidermal cells, retinal cells, the liver, and the thymus [7, 87]. Whether or not these extra‐pineal sites of melatonin production play a role in circadian regulation comparable to that of the pineal tryptophan‐serotonin biosynthetic pathway is not yet known.

There are two identified receptors of melatonin, MT1 and MT2, which are of a class of receptors known as G‐coupled protein receptors, meaning that they are involved in several different signal transduction pathways [89]. The MT1 receptor utilizes its G‐protein coupling to regulate the inhibition of adenylyl cyclase and activation phospholipase C to transmit signals upon melatonin binding, whereas the MT2 receptor uses its coupled G‐proteins to inhibit both adenylyl cyclase and and soluble guanylyl cyclase to transmit relevant information when bound to by melatonin [90]. It should be noted that MT2 receptors bind to endogenous melatonin with a significantly lower affinity than MT1, so MT2 is of much less relevance to circadian regulation and will receive significantly less attention in the discussion to come [89].

Exogenous melatonin refers to any melatonin ingested in the form of a supplement or administered via therapeutic treatment [84]. For this reason, the efficacy and mechanisms associated with exogenous melatonin will be examined closely in the section that deals with potential therapeutics for circadian dysregulation later in this review. For now it is worth noting that there are limitations to the uptake and thus, the efficacy of exogenous melatonin, making it distinctly different from endogenous melatonin in terms of its influence on circadian rhythm [91].

There is an established association between disrupted melatonin synthesis and/or secretion and pathological ramifications. While this article will discuss circadian dysregulation in relation to AD, there is also an observed correlation between decreased melatonin synthesis and other neurodegenerative diseases, dermatological ailments, cardiovascular diseases, psychiatric diseases, vascular diseases, erectile dysfunction, and several different types of cancer [7]. Even more importantly, nearly all common and uncommon sleep disorders involve dysregulation in the synthesis and/or secretion of melatonin, including sleep apnea, restless leg syndrome, narcolepsy, and insomnia [7, 92].

5 Cortisol

Almost every cell in the body has the potential to be affected by cortisol, making it one of the most potent endogenous substances in the human body [93]. In the context of circadian regulation, cortisol is largely responsible for the promotion of wakefulness, and is in many ways the opposite to the sleep promoting hormone melatonin [3]. Cortisol levels are observed at their lowest abundance in the early stages of sleep and remain low until just before waking [73].

The synthesis of cortisol occurs at the hypothalamic pituitary adrenal (HPA) axis, which is, in essence, an amalgamation of endocrine pathways responsible for the mediation of negative feedback loops that involve the hypothalamus, pituitary gland, and adrenal gland [94]. Cortisol, as a corticosteroid, is synthesized from cholesterol [95]. On demand, pregnenolone makes use of a multitude of cytochrome P450 enzymes to derive cortisol from cholesterol [95]. Once formed, cortisol is secreted into the bloodstream. As mentioned, this secretion will peak in the morning hours to promote wakefulness, plateau throughout the day, and trend downwards about two hours before an individual falls asleep, reaching its low point during the sleep state [96].

Secretion timing in response to SCN‐related triggers is critical to understanding the role that cortisol plays in the regulation of the circadian rhythm [11]. Cortisol is secreted by the adrenal gland on demand, meaning that it is not stored after being synthesized and released at a later time [93]. As such, the timing of cortisol synthesis and secretion is under the tight control of the master oscillator in the SCN, as well as a peripheral oscillator in the adrenal gland, which will stimulate secretion of cortisol in response to the secretion and reception of the adrenocorticotropic hormone (ACTH) [97].

When the HPA axis activity is heightened in response to physiological activity or some type of psychological stressor, it will induce the secretion of the corticotropin‐releasing hormone (CRH) and arginine vasopressin (AVP) from the hypothalamic paraventricular nucleus [94]. These two hormones, the latter of which is a key modifier of SCN function as mentioned earlier, will bind to their respective receptors, CRH‐R1 and V1B, in the pituitary gland, and this bondage will in turn trigger the secretion of ACTH [93, 94]. When released into the bloodstream, ACTH will trigger the synthesization and secretion of cortisol from the adrenal gland [93].

Interestingly, cortisol abundance in the body has been shown to have an effect on the secretion of CRH and ACTH. Low levels of cortisol within the bloodstream will trigger an increase in CRH and ACTH secretion. This mutual abundance modification evinces a regulatory relationship and speaks to an equilibrium that the HPA axis works to maintain [93, 98].

The fact that cortisol is such a potent hormone throughout the body, means that it serves many different functions, aside from just circadian regulation [93]. This hormone can be more broadly characterized as a hormone that influences arousal, meaning that it is heavily involved in stress and fear, as well as biophysical regulation [99]. For example, the well known “fight or flight” response to a dangerous situation, involves the release of adrenaline and norepinephrine in conjunction with cortisol. While the former two hormones are used to heighten the physical response to a threat over a short period, cortisol is the longer acting hormone that will sustain the cognitive representation of that threat [100].

Cortisol’s role as a mediator of the stress response and general arousal can have adverse effects on circadian regulation. Given the fact that psychological traumas and chronically stressful situations are known to upregulate cortisol synthesis and secretion, leading to a sustained increase in blood‐cortisol levels over a longer duration of time, it follows that these factors will play a contributory role in the propagation of pathological wakefulness [101]. Therefore, the inability to manage stress can directly dysregulate circadian function by upregulating the rate and quantity of cortisol secretion, which can contribute to the exacerbation of sleep disorders such as insomnia [11]. Furthermore, numerous studies have shown that there is a strong correlation between cardiac irregularities, the impairment of adaptive abilities, dyslexia, ADHD, and can be toxic to the hippocampus [102–104].

6 Serotonin

The synthesis of serotonin is a multistep process, beginning in the terminals of serotonergic neurons, L‐tryptophan is hydroxylated by tryptophan hydroxylase (Tph), converting it into 5‐hydroxytryptophan (5‐HTP) [105, 106]. 5‐HTP is then catalyzed by aromatic amino acid decarboxylase (AAAD) into serotonin [106]. Unlike cortisol, serotonin can be stored after it is synthesized, typically in serotonergic neurons, specific gastrointestinal cells, and in blood platelets [107]. Upon demand, stored serotonin is generally released into the synapse, where it will bind to and elicit a nerve impulse on the postsynaptic neuron [106, 108]. That said, it is also known to bind to presynaptic autoreceptors, to facilitate further synthesis and secretion of serotonin [109]. Serotonin synthesis and release can occur in serotonergic neurons throughout different areas of the brain, and is best known for being in the pineal gland, however in the context of circadian rhythm, the raphe nucleus and its associated serotonergic neurons are of the most relevance [58].

Recall that one of the critical elements for the proper function of the SCN and optimization of circadian rhythmicity, are the functional afferent projections to the SCN that receive sensory input, primarily from retinal ganglion cells that perceive and transmit photic inputs to regulate levels of melatonin and cortisol [54, 59]. Afferent projections from the raphe nuclei play just as important a role for the regulations and entrainment of optimal circadian rhythms as the photic afferents from the retina [54].

The SCN contains what is referred to as a serotonergic terminal plexus, which is the point in the SCN where afferent projections from neurons within the raphe nuclei terminate, such that serotonergic information passed from the raphe nuclei can act on cells within the shell and core of the SCN [58]. In addition, the aforementioned intergeniculate leaflet (IGL), which itself acts as a peripheral oscillator, receives afferent projections from the raphe nuclei [58, 61]. To understand the role that these projections from the raphe nucleus play in terms of serotonergic innervation within the SCN, it is important to note that the raphe nucleus can be subdivided into sections: the dorsal raphe nucleus (DRN) and medial raphe nucleus (MRN) [110]. Neuroanatomical studies involving hamsters have revealed that the DRN contains afferent neurons which project to the IGL, but not the SCN, whereas the MRN contains neurons which project to the SCN, but not the IGL [56]. This indicates that the master oscillator (SCN) and one of several peripheral oscillators (IGL) are independently innervated by these two different sections of the raphe nucleus, and therefore their role in serotonergic signaling will have two different, equally important effects [58, 61].

It is also important to note that in either case, not all projections from the raphe nucleus to the SCN are serotonergic in nature [111]. In fact, one study used immunohistochemical methods of neuroanatomical evaluation to show that only about 50% of the afferent projections from the MRN to the SCN are serotonergic, and only 40% of those afferent projections from the DRN to the SCN are serotonergic [112]. The role that the nonserotonergic projections play is unknown thus far [113]. In addition, the serotonergic innervation of nuclei throughout the brain, especially in the mid‐ and forebrain, are vast [54]. While there is no evidence that links these afferent serotonergic projections to other non‐SCN/IGL nuclei within the brain to circadian regulation, future studies may reveal an unseen connection between these seemingly unrelated brain areas, and serotonergic regulation of circadian function [58, 61].

Afferent projections from the raphe nucleus to the SCN and IGL extend over the same space as the afferent projections of the RHT and GHT, which aid in the regulation of melatonin and cortisol signaling [54, 114]. However, they have also been shown to be involved with serotonergic signaling and innervation along with raphe projections [54]. Morin (1999) notes that there is a great deal of synaptic contacts between these signaling tracts, as well as shared targets within the SCN and IGL [112].

Numerous evaluation models have been employed to study the effects of serotonin on circadian rhythm. Among these modalities is electrical stimulation of differing neuroanatomical regions. Using this type of study, it has been observed that DRN or MRN activation will elicit the release of serotonin in the SCN [115]. Furthermore, there has been an observed correlation between this electrically induced serotonin release and phase shifts in the circadian rhythm. In vitro studies have shown that increased serotonin levels tend to advance the phase shift of the circadian rhythm [116].

Raphe nuclei, like the SCN, receive several retinal afferent projections, meaning that the process by which serotonergic innervation of the SCN and IGL induce phase shift changes is likely driven by light exposure [54, 58]. Support for this proposed medium of serotonin activity on the SCN is found in several studies that have shown that timed exposure to bright lights will induce activity in serotonin antagonists, which will subsequently advance circadian phase shift [4, 114]. In addition, several studies have shown that there is a relationship between glutaminergic input to the SCN in response to light, and serotonin levels and activity, indicating that serotonin’s role in the SCN regulation process may be as a signaling mediator between photic inputs and glutaminergic activity in the SCN [58, 117, 118]. This is supported by the fact that serotonin is instrumental in the potentiation of glutamate, which can act on core neurons in the SCN to promote excitability, increased firing rate, and wakefulness during the sleep period of the sleep–wake cycle [64].

The mechanisms of serotonergic influence on the SCN, IGL, and other peripheral oscillators is not yet fully understood. While it is known to be a significant modulator of circadian activity and a driver of circadian phase shifts, the physiological mechanisms and specific signaling cascades that allow it to serve those regulatory functions remain elusive. Due to the fact that the role of serotonin is much more ambiguous than that of melatonin and cortisol, it is difficult to speak with any level of certainty about the role that serotonergic disruption may play in pathological dysregulation of circadian function, not to mention its relationship to AD [119, 120]. Despite these difficulties, it is important to recognize that serotonin is one of the major drivers of circadian optimization in the healthy circadian rhythm, and thus, it is likely a very dangerous propagator of pathological abnormalities in a dysregulated circadian rhythm. As such, it should be studied much more extensively to elucidate the specific physiological mechanisms that drive its observed phase regulation functions.

7 Other biomolecular factors involved in circadian rhythm

Different hormones, neurotransmitters, and neuromodulators involved in circadian regulation can be divided amongst the pathways in which they principally operate. Recall that there are generally three pathways of interest in the context of circadian rhythm: geniculohypothalamic tract, retinohypothalamic tract, and the raphe projections. That said, intra‐SCN communication biomolecules which operate within efferent projections from the SCN and peripheral oscillators should also be accounted for.

Histamine acts on the RHT in a manner similar to serotonin, in that elevated levels have been correlated to advanced phase shifts [58, 121]. In vitro studies have shown that a heightened abundance of histamine binds to H1 receptors at the SCN to promote excitation, and conversely, when histamine binds to the H2 receptor at the SCN, it is shown to inhibit neuronal firing within the SCN [122]. Histamine could potentially be a highly influential amine in terms of circadian regulation, however study on its effects are limited and more research is needed to further elucidate its role in regulating the function of the SCN and peripheral oscillators [58].

Nitric oxide (NO) is a highly involved RHT retrograde messenger in terms of photoreceptivity within the afferent projections from the ganglion in the retina to the SCN [123]. Studies have shown that blocking the production of NO in hamster models impaired the transmission of light along these retina‐SCN afferent pathways [123, 124]. Furthermore, this NO production blockage impaired glutamate release in SCN retinal terminals [58]. Because serotonin potentiates this release, it is possible that there is interaction between serotonin and NO in the glutamate potentiation‐release process, and reduction in NO will limit serotonin’s ability to potentiate that release [58, 72].

Neurotensin is an RHT neuromodulator that has been shown to increase the frequency of neuronal discharge in the murine SCN when it is abundant [125]. The SCN possesses two neurotensin receptors, NTS1 and NTS2, indicating that it does indeed play a mediatory role in cellular signaling [58, 125]. Further study in mouse models has shown that neurotensin acts in opposition to the SCN neuronal inhibitor NPY, in that it diminishes the inhibitory effect of NPY when its abundance is increased [58].

Gastrin releasing peptide (GRP) is an RHT peptide whose receptor BB2 is synthesized within the neurons of the SCN [126]. Studies have shown that GRP is involved in photic entrainment, meaning it is a mediator of photic inputs from the afferent projections of the retina [126, 127]. While the specific regulatory role that it plays is unknown, it has been shown in animal models that upon administration of GRP, SCN neuronal firing was decreased during early night and increased during late night, whilst having no effect on neuronal firing during the day [62, 126]. This could imply that GRP plays a role, similar to that of glutamate, in the maintenance of the sleep period.

Neuromedin S (NMS) is an RHT neuropeptide that has receptors in the core of the SCN, and has been shown to increase neuronal firing within the SCN [53, 62]. While study on this peptide is limited, it appears that it acts non‐photically, which if true, means that it is a purely endogenous regulator of the SCN [62]. This would likely mean that it plays some sort of mediatory role and interacts closely with one or several different neurotransmitters and hormones to exert its function. Given the fact that increased abundance of NMS is correlated with advanced phase shift, it is possible that it plays a role in serotonergic signaling and/or interacts closely with histamine.

The aforementioned GHT orexigenic peptide neuropeptide Y (NPY) plays an important inhibitory role on the discharge rate of SCN neurons [54]. Hamster models have shown that NPY acts directly on pacemaker neurons in the SCN [58]. In addition, NPY has been shown to be highly involved in projections to and from the IGL and ENK peripheral oscillators, where it helps to mediate immunoreactive responses to experimentally induced lesions in these peripheral oscillators. It also interacts closely with GABA [128]. NPY colocalizes with GABA, and acts presynaptically to inhibit the firing of GABAergic neurons in the SCN by limiting the flow of calcium currents [128, 129]. NPY administration has also been shown to induce phase shifts during daylight hours, indicating that it plays a role beyond immunoreactivity, likely as an inhibitor of SCN neurons associated with the sleep period [58, 130].

The RHT neurotransmitter acetylcholine (ACh), appears to play a modulatory role in the reception and interpretation of afferent photic inputs [131]. Because it has been shown that it does not play a direct role in the photic input pathway, as it was once thought to, ACh is now thought to play some role in the modulation of light information received from the retina [4]. ACh has been shown to alter the length and regularity of circadian cycles when administered in murine models. When triggered by an intracerebral injection of one of its agonists, Carabochel, in the brains of mice, the induced circadian phase shifts in response were very similar in their effects and consequences to a brief flash of light [132].

γ‐amino butyric acid (GABA) is a GHT neurotransmitter that is highly involved in the regulation of neuronal discharge within the SCN. Given that nearly all SCN neurons have been shown to express GABA, and its synthesization enzyme glutamic acid decarboxylase (GAD), it is no surprise that GABA is involved in both daytime excitation and nighttime inhibition of SCN neurons [133, 134]. GABA depolarizes neurons selectively in response to photic stimuli [58, 135]. In response to light, depolarization will be limited, thus allowing for neuronal excitation [58, 135]. In response to darkness, depolarization will increase, causing an inhibition of neurons associated with wakefulness, in order to help promote sleep [58, 135]. As mentioned, GABA interacts closely with NPY, which presynaptically inhibits the firing of GABAergic neurons in response to photic input, meaning that it is likely involved in the daytime inhibition of sleep related GABAergic neurons [136].

Vasoactive intestinal polypeptide (VIP) is a peptide whose receptors are abundant in the core of the SCN [137]. They are responsive to photic input from the afferent projections in the retina, and have been shown to be instrumental in the resynchronization of a dysregulated circadian rhythm [58]. Several studies have shown that experimentally dysregulated circadian rhythms in murine models have been restored to optimal or near optimal sleep–wake patterns upon the administration of VIP [62, 138]. This implies that a pertinent, and likely common, cause of circadian dysregulation is a decrease in VIP. In addition, VIP is involved in the maintenance of both sleep and wake periods [139]. One study showed that VIP induces expression of the PER1 and PER2 genes in accordance with the period of the circadian cycle, and both of these genes are involved in the maintenance of sleep and wakefulness periods [140].

Somatostatin (SS) is a hormone that is active in efferent projections from the SCN, and neurons which express SS are present in both the core and shell subregions of the SCN [141]. One study showed that there is an increase in the expression and activity of SS and an associated peptide, substance P (SP), in response to experimentally induced lesions in aged mice, which may indicate that there is no decrease in SCN peptidergic immunoreactivity with age [121]. However, a similar study involving aged hamsters yielded opposite results, indicating that the findings of Biello (2009) may not translate to other species. It also appears that SS may play a mediatory role in the signaling cascades that contribute to phase shifting, as it interacts closely with neurons expressing AVP and VIP [142].

Vasopressin (AVP) is a nonapeptide involved in intra‐SCN communication, and it is instrumental in neuronal excitation during wakefulness [143]. AVP activates V1a receptors within the core of the SCN, which triggers an increase in neuronal discharge [58]. Like VIP, AVP is widely expressed on neurons within the core of the SCN, making it highly influential in the regulation of circadian signaling cascades [54]. Induced reductions in AVP expression in murine models has shown a decrease in time spent running on a wheel [58]. This indicates that AVP, because of its role in SCN neuron excitation and promotion of wakefulness, plays a role in the promotion of locomotion and physical activity. It is likely that, based on these murine models, individuals who are generally regarded as more active, have a heightened expression of AVP in SCN neurons, than individuals who are less physically active, although this is purely an assumption and there may not be a correspondence between these observations in mice and similar characteristics in humans.

Calbindin (CalB) is a protein involved in efferent projections from the SCN that, much like SS, has been observed in both the core and shell subregions of the SCN. CalB expressing cells are photosensitive, and likely play a role in the transmission of photic information to peripheral oscillators along the efferent projections from the SCN. It has also been shown that CalB expressing cells are highly connected to neurons which express other previously mentioned circadian‐regulating factors including AVP, GRP, CCK, NPY and VIP [54]. While the strength of these connections varies, CalB’s breadth of connection indicates that it may be important for signaling to a wide variety of neurons responsible for the regulation of circadian excitation, inhibition, and maintenance [54, 58]. Given the fact that it is photosensitive, it is possible that CalB acts as a prime transmitter for photic information that informs the activities of these connected neurons and their respective neurotransmitters [58]. In addition to these intra‐SCN connections, there is a high degree of connectivity between CalB expressing cells and extra‐SCN brain regions, further implying that CalB could be a primary signaling protein for photic stimuli [144]. Studies have also shown that in the absence of photic stimuli, CalB expressing cells decrease in activity, which inhibits certain signaling cascades associated with wakefulness [144, 145].

Calretinin (CALR), Met‐Enkephalin (mENK), Prokineticin‐2 (PK2), and Angiotensin II (ANG‐II), are all proteins and peptides that act along the efferent projections from the SCN, although their specific functions in terms of circadian regulation are not well understood [58]. Neurons expressing these proteins have all been observed in relatively small abundance in the SCN, so they likely play some role in circadian regulation. ANG‐II is theorized to act as a modulator of some kind for efferent signaling and angiotensin‐1 receptors on the endothelial plasma of the SCN parenchyma have been observed, but no experimental results have substantiated this hypothesis. mENK expressing neurons have been shown to display withdrawal‐like responses when endogenous enkephalins are reduced or removed, indicating that enkephalin and other endogenous opioids may play a role in modulating SCN function, and that this role is mediated through the mENK peptide [58, 146]. However, these findings and their potential significance are cursory at best. PK2 is thought to be of particular relevance to nocturnal species, and is said to be involved in the inhibition of locomotor activities [58, 147]. Further study of these circadian‐regulating factors is needed to elucidate the specifics of their respective functions.

8 Connections between AD and circadian dysfunction

As mentioned, our understanding of both circadian rhythm and AD are incomplete, however significant study in recent years has shed light on the relationship between these two ailments, elucidating potential interactive mechanisms and novel avenues for the development of therapeutics.

The most pertinent observation speaking to the relationship between AD and circadian disruption is the fact that the size of the suprachiasmatic nucleus has been shown to decrease with dementia [1, 128]. Along with this decrease in structural size, comes a loss in the expression of VIP, AVP and neurotensin, all of which are aforementioned regulatory factors of circadian function [62]. Interestingly, it has also been shown that these losses in SCN structure and reductions in regulatory hormone level not only occur, but occur in the very early stages of dementia onset [148, 149]. Non‐AD related studies have shown a correlation between dysregulation of the neuron to glia ratio in the SCN, and circadian dysfunction [150]. More recent studies have shown that neurodegeneration of the SCN aids in the dysregulation of its neuron to glia ratio, and this pathological process likely plays a role in the loss of circadian function with AD progression [151–153].

Through postmortem tissue analysis, Harper et al. observed that circadian dysregulation is proportional to AD progression, as measured by Braak stage [154]. This finding was highly influential, as it evidenced a direct correlation between AD progression and circadian dysregulation.

The cholinergic basal forebrain, a brain structure from which the SCN receives afferent projections, is observed to undergo rapid neurodegeneration in the AD brain [155]. This peripheral oscillator of circadian rhythm has received little study pertaining to its role in circadian regulation, however it is known that cholinergic innervations received by the SCN from the cholinergic basal forebrain act on cells within the SCN, meaning that it likely plays some role in the regulation of circadian function [54, 58, 155]. Wisor et al. utilized transgenic mice to show that disruptions in the cholinergic basal forebrain’s signal transmission to the SCN dysregulated the non‐rapid eye movement phase of the sleep period [156]. Furthermore, experimental lesions to this region have been shown to induce adverse phase shifts and impair the restoration of a dysregulated diurnal cycle [157]. While the specific process which allows the cholinergic basal forebrain to regulate the SCN remains unknown, the fact that it has been observed to play a role in this regulation and is subject to rapid degeneration in the AD brain, makes it a fertile area for future study.

Some studies have shown relevant correlations between circadian dysregulation and certain hallmark physiological symptoms of AD. Transgenically modified mice have been shown to have peak Aβ concentrations during wakefulness, and similar results have been found in the cerebrospinal fluid (CSF) samples collected from human AD patients [158]. Interestingly, there is a negative correlation between the duration of the sleep period and ISF Aβ [159]. This could show that the restorative benefits of sufficient sleep help to regulate Aβ concentrations, and sleep deficiency caused by dysregulation of circadian rhythm as a consequence of AD pathology, may play a role in exacerbating amyloidogenic cleavage, thus driving the progression of AD [160]. Sleep has been shown to increase ISF by nearly 60%, and this will increase glymphatic flow and clearance of toxins [161]. Furthermore, the levels of several variants of lymphatic drainage which are simply CSF fluids containing lymphocytes and other immune cells, are drastically increased during sleep [162]. If sleep duration and/or quality is impaired in an AD patient, the clearance of these toxins and restoration of damage that they may have incurred, will be insufficient to stave off the accumulation of Aβ and and other pathological toxins in the AD‐brain, thus making circadian dysregulation, and consequent sleep deprivation, driving mechanisms for the progression of AD [26]. More study is needed to confirm this.

Melatonin production has been observed to be markedly downregulated in the early stages of AD as well, and this undoubtedly impairs the optimal functioning of the circadian clock [10]. Moreover, melatonin downregulation has been strongly correlated to upregulated deposition of Aβ and its accumulation into senile plaques [163, 164]. Studies pertaining to the relationship between amyloid deposition and the downregulation of endogenous melatonin have implicated the default mode network (DMN), a region of the cerebral cortex whose connectivity is critical for proper cognitive function [165]. Connectivity in the DMN varies in accordance with sleep patterns. In the healthy brain, connectivity has been observed to decrease during short wave sleep, possibly to aid in the consolidation and long‐term storage of memories [166]. Positron emission tomography (PET) studies have shown that DMN connectivity is heightened during poor sleep, and, in AD and MCI subjects, it is in this poor sleep state that Aβ deposition is upregulated [158]. Self‐reported measures of sleep quality and duration as measured by the Epworth Sleepiness Scale (ESS), have revealed a strong correlation between AD severity and both poor sleep quality and short sleep duration, and this likely has something to do with heightened DMN connectivity during short wave sleep [158, 167].

Because fMRIs have shown that DMN connectivity patterns are also dysregulated in non‐AD subjects who self report poor quality or short duration sleep, it is reasonable to assume that melatonin downregulation leading to insomnia and other sleep disorders, precedes sustained DMN connectivity during the idealistic sleep period, which then leads to increases in the amyloidogenic cleavage of APP to heighten Aβ peptide deposition [10]. If true, this process would fuel a cycle in which the ratio of endogenous melatonin to Aβ is progressively dysregulated, simultaneously fueling the progression of AD and worsening circadian dysregulation.

Although the specifics of the interaction between DMN connectivity and Aβ deposition are not fully understood, studies have revealed that there is a noteworthy correlation between the two [168, 169]. Most importantly, a more significant abundance of senile Aβ plaques can be observed in the atrophied DMN region during late stage AD than in virtually any other area of the brain [10, 170, 171]. This implies that the DMN region may be an epicenter for early AD development. This assumption is backed by the observation that DMN connectivity patterns in MCI, the early stage precursor to AD, show DMN connectivity dysregularities similar to those observed during fully realized pathological AD [170]. This consistency of increased DMN connectivity during sleep, is indicative of the fact that downregulated melatonin production and other circadian irregularities that cause poor and/or short sleep, by extension, facilitate the progressively amplified deposition of the Aβ peptide. For this reason, decreased DMN connectivity as a result of early circadian dysregulation, are not only correlated with AD onset and progression, but may play a causal role in the onset of MCI and its transition into fully realized AD. More study is needed to further elucidate the interplay between melatonin downregulation, increased DMN connectivity, and Aβ peptide deposition, but it can at least be assumed that circadian dysregulation and increased DMN connectivity during sleep precede the pathological deposition of Aβ in the DMN regions of the cerebral cortex.

There is also a connection to be made between the findings related to DMN connectivity irregularities and the impaired restorative function of sleep in the AD brain. As previously mentioned, short duration and poor quality sleep have been firmly associated with decreased levels of CSF and ISF fluids associated with the removal of toxins from the brain. Circadian dysregulation leading to heightened DMN connectivity during the ideal sleep period, will promote arousal that will impair glymphatic flow and the secretion of these fluids [10]. For this reason, it is entirely possible that increased DMN connectivity during sleep does not directly cause Aβ deposition, but instead promotes a state of unnecessarily heightened arousal during the sleep period, thus impairing the mechanisms which would otherwise clear Aβ and other toxins from the DMN and other brain regions. If this is the case, increased DMN connectivity would not necessarily cause the increased deposition of Aβ, but would simply block its clearance by inhibition of restorative CSF and ISF lymphatic fluids secreted during sleep, hence allowing for Aβ accumulation over time.

Decreased expression of melatonin receptors in the SCN and peripheral oscillators have also been observed in the AD brain [7, 172]. Taken at surface level, this observation implies a reduced uptake of endogenous melatonin which will impair the efficacy of melatonin processes in the circadian regulatory mechanism [173]. While this is indeed the case, the prevalence of the MT1 melatonin receptor and the expression of MTNR1A, the gene from which it is derived, appear to have a more intricate role in diurnal regulation and the circadian dysregulation observed in the AD brain [3].

Sulkava and colleagues showed that experimental reduction of the MT1 receptor in vitro, led to more abundant amyloid deposition. This implies that APP metabolism is regulated by circadian components. To extend this observation to human subjects, Sulkava and her team studied a previously established genetic risk variant, rs12506228, which has been associated with the gene MTNR1A, whose expression likely dictates the prevalence of the MT1 melatonin receptor. Two cohorts, one composed of longtime shift workers with chronic, self‐reported sleep deprivation, and another made up of older adults who had been diagnosed with AD, showed similar patterns of circadian dysregulation. In both populations, expression of the risk variant rs12506228 was associated with a substantial decrease in MT1 receptor abundance at the SCN. In the population of AD patients, there was a marked increase in Aβ deposition and accumulation, inversely proportional to the downregulated MT1 melatonin receptor. In addition, decreased abundance of MT1 melatonin receptors diminished or outright inhibited melatonin’s ability to act on the SCN and serve its regulatory purpose for the greater circadian rhythm. Taken together, these findings indicate that a downregulation in MT1 melatonin receptors will facilitate an increase in Aβ deposition and dysregulated circadian function [174].

Interestingly, experimental studies have shown no upregulation in βACE‐1 abundance in the SCN and peripheral oscillators, meaning that the propensity for APP in the SCN to be cleaved along the amyloidogenic pathway while the circadian rhythm is in a dysregulated state may be directly influenced by the abundance of melatonin receptors within the SCN, rather than coming as a result of increased βACE‐1 levels [175, 176]. If this is true, it means that by some mediatory mechanism, lower prevalence of MT1 receptors induces cleavage along the amyloidogenic pathway, hence the association between decreased MT1 receptor levels, and increased Aβ deposition. It is possible that MT1 abundance, while not increasing the amount of βACE‐1 in the SCN, may increase its activity. Conversely, it is possible that the activity or abundance of α‐secretase is diminished in response to lower MT1 receptor availability. Either of these changes in the SCN would make the cleavage of APP along the amyloidogenic pathway more likely, however more study is needed to verify this idea.

In addition to the role of melatonin receptors and their underlying genetic components in regulating circadian rhythm and driving AD pathology when in a dysregulated state, melatonin is also known to serve a neuroprotective role [177]. Several studies have elucidated the fact that this hormone acts a scavenger of free radicals, thus aiding in the clearance of toxins from the brain and the immune system‐associated restoration effect that occurs during sleep [167]. MT1 receptor prevalence is correlated to the efficacy of the neuroprotective apoptotic effect along Rip2/Caspase‐1 and mitochondrial apoptotic pathways [177].

Given the fact that melatonin is a well established antioxidant, it has a great effect on the scavenging of radical oxygen species (ROS), radical nitrogen species (RNS), and the OH free radical. This hormone has also been shown to elicit the expression of several genetic components which produce both prooxidant enzymes and antioxidant enzymes [48]. Among these are the genes glutathione peroxidase (GPx) and glutathione reductase (GRd), the latter of which increases the amount of the oxidized form of glutathione (GSSG) [48, 178]. In response to heightened oxidative stress, GPx will stimulate increased production of GSSG. While the specifics of its role are not well understood, GSSG in the healthy brain is relatively low, thus its increased abundance is a relevant biomarker for many cognitive diseases, including AD. GRd on the other hand is an enzyme which reduces GSSG to the neuroprotective GSH, and thus the expression of GRd is a critical component of diseases, like AD, which induce oxidative stress and increase GSSG levels [48, 178, 179]. Melatonin has been shown to act on two enzymes, γ‐glutamylcysteine synthase and glucose‐6‐phosphate dehydrogenase, which increase the synthesis of GSH from GRd [48, 178, 179]. This process, when effective, restores the optimal ratio of GSH to GSSG, thus reducing the abundance of ROS and increasing the abundance of the neuroprotective GSH [48, 179]. For the reasons aforementioned, melatonin is an important mediator of this regulatory redox reaction. But, decreases in the abundance of melatonin and its receptors will dysregulate this immune process, which will impair the brain’s ability to protect against oxidative stress. Because oxidative stress is a symptom and driving factor of AD, decrease in the synthesis and secretion of melatonin or the abundance of its receptors as a result of circadian dysregulation, will exacerbate oxidative damage associated with AD.

In the healthy brain, melatonin inhibits the binding of NF‐κB to DNA, which in turn downregulates the production and abundance of proinflammatory cytokines through the inhibition of inducible nitric oxide synthase (iNOS) and cyclooxygenase (Cox) genes [43, 180, 181]. In the AD brain, NF‐κB dysregulation is a near universal pathophysiological symptom, and this facilitates an upregulation in the secretion of proinflammatory cytokines including IL‐1β, IL‐6, and TNF‐α among others [43]. Presumably, downregulation in melatonin and melatonin receptor availability will lead to increases in NF‐κB‐DNA binding, thus driving the production of cytokines and exacerbating AD‐related neuroinflammation [182]. Dysregulation in the circadian processes which modulate the timing and frequency of melatonin synthesis and secretion, can therefore lead to increases in AD‐related neuroinflammation.

For the reasons thus far discussed, it is safe to assert that melatonin is not only a circadian regulatory hormone, but also a hormone whose activity and abundance may be critical to the immune system. Therefore, circadin dysregulation that diminishes the level of melatonin and its receptors both in the SCN and beyond, may lead to immune deficiencies that contribute greatly to the onset and progression of AD.

Chronic elevation in cortisol levels over time are strongly correlated to the onset of preclinical MCI and AD [183]. Several studies have established that experimental glucocorticoid treatment in murine models results in an increase in Aβ deposition and the phosphorylation of the tau protein [184–186]. In addition, heightened atrophy of the hippocampus, and common behavioral ramifications such as memory deficits have also been observed [185, 187]. Furthermore, cortisol circulation in human subjects appears to be upregulated as many as six years prior to AD diagnosis, indicating that it plays a role in the onset of preclinical MCI and possibly mediates its transition into clinical scale AD [188].

Studies pertaining to cortisol often involve the measurement of urinary free cortisol (UFC) which is a cortisol measurement that isolates the active cortisol hormone without its examination being clouded by other particles and/or substrates [189]. Ennis et al. (2017) used UFC/creatinine (Cr) ratios, as measurement for AD severity in a sample of human subjects to show that subjects whose 24 hour UFC/Cr ratio was variable over time, were at the greatest risk for AD [183]. Conversely, subjects who had consistent UFC/Cr ratio outputs over time, whether they be high or low, were at a lesser risk. Preclinical dysregulation in the neurons and glial cells of the hippocampus could be the cause of preclinical fluctuations in UFC/Cr ratios, which represent variable cortisol synthesis and secretion over time, independent of stressors [183]. Because the hippocampus regulates cortisol secretion through the inhibition of the HPA axis after cortisol levels are elevated in response to acute stressors, its degeneration could prevent it from serving this regulatory function in clinical AD patients, leading to sustained cortisol levels over time.

Other prospective studies pertaining to cortisol levels and AD onset and progression, have found no relationship between cortisol levels and MCI or AD [183, 190]. For this reason, the relationship between cortisol and AD is not as firmly established as the relationship between melatonin and AD. Certain findings have indicated that it may play a potential role, so this relationship is in need of more study.

Serotonin is a known regulator of depression and other behavioral disorders, many of which are considered modifiable risk factors for AD [191]. Certain physiological consequences of depression, such as heightened activation of the HPA axis, the hypersecretion of glucocorticoids, and hippocampal atrophy, are all physiologically relevant to AD pathology as well, indicating that there is likely a correlation between the two [191–193]. Studies have shown that late onset depression can presage MCI symptoms, with older populations being significantly more likely to exhibit dementia‐like behaviors within five years of depression onset [194]. Moreover, there is a noteworthy correlation between the preclinical deposition of the Aβ peptide, and depressive symptoms, with heightened Aβ deposition making an individual four times more likely to develop depressive symptoms [18].

There is a strong correlation between depressive symptoms and circadian rhythm dysregulation [157]. Two common variants of depression, unipolar and bipolar depression, have relatively consistent circadian dysregulations as characteristics [195]. In unipolar depression, patients have a tendency to report feeling more positive in the afternoon than in the evening and night, and the converse is true for bipolar depression. In both of these depression variants however, there is generally observed insomnia during the sleep period and hypersomnia during wakefulness [195, 196]. These circadian irregularities common in depressed individuals, only scratch the surface of the circadian system as it relates to depression, however, in terms of its relationship to AD, it is important to note that circadian dysregulation as a result of the depressed state, is thought to be a consequence of irregular serotonin synthesis and secretion [157, 197].

Depressed patients suffer not only from ill timed insomnia and hypersomnia, but also from physiological consequences such as metabolic dysregulation and impaired mood regulation, and these are known to be mediated in large part by serotonin signaling pathways [198]. Serotonin is known to suppress appetite, as well as promote physical activity [199]. Furthermore, serotonin plays a key role in regulating the way in which food is metabolized. It circulates in the bloodstream and acts on metabolic and digestive tissues in organs to promote the secretion of other metabolic hormones such as insulin, as well as regulating energy storage in brown and beige adipose tissue, whose optimal function is critical to the regulation of fat storage and, by extension, body mass index [199]. In addition, optimal serotonin levels have also been attributed to heightened focus, emotional stability, and feelings of “happiness” or “calmness”, making it a key regulator of an individual’s mood [200]. It also plays a role in the regulation of cellular homeostasis, respiratory regulation, cell growth, immunal optimization, heart rate, and vascular tone [199, 200].

For these reasons, it can be concluded that serotonin signaling is important for the optimization of both circadian regulation, as well as overall physical health. While little is known of its effects on AD in the context of the circadian rhythm, dysregulated serotonin levels appear to drive circadian dysregulation, which in turn has been shown to lead to immunal comprimization and greater Aβ deposition in the preclinical MCI stage. This could explain the correlation between depressive symptoms, likely induced by serotonin synthesis and secretion dysregulation (among other cofactors), and the increased likelihood to develop AD. More study is needed to firmly establish the mechanisms that cause this correlation.

Other physiological mechanisms are of importance to the relationship between AD and circadian dysregulation, and many of these occur in extra‐SCN peripheral oscillators scattered in various organs and tissue throughout the body [201]. As mentioned, circadian dysregulation leads to immunal deficiencies, and several of these deficiencies deserve further exploration. In the optimally functioning circadian sleep/wake cycle, the wakefulness phase is characterized by the increased activity of leukocytes, and heightened levels of circulating epinephrine, norepinephrine, and TNF‐α [202]. The sleep phase on the other hand is characterized by increased glymphatic flow, production of immune cell rich ISF and CSF fluids, and a lower expression of endothelial adhesion molecules [202, 203]. These roles for both wake and sleep phase of the optimized circadian rhythm are protective, in which inflammation is increased during wakefulness to aid in the short‐term treatment and maintenance of sites where minor injuries may occur throughout the day, and the sleep period is meant to heal these injury sites and quell inflammation throughout the night. Disrupted circadian timing, specifically circadian dysregulation characterized by insomnia and sleep deprivation, can facilitate a sustained upregulation in cerebral and bodily inflammation, a factor known to contribute to both the onset and progression of AD.

Sustained neuroinflammation in the AD brain is known to lead to an increase in the activity of microglia and astrocytes, which are immune cells involved in tending to neuronal lesions [51]. In the AD brain, microgliotic and astrogliosis inflammation is known to be upregulated at sites where Aβ deposition and NFTs are most abundant, so the severity of microgliosis and astrogliosis appear to increase throughout the course of AD’s progression [204]. Both microglial and astroglial cells are activated by proinflammatory cytokines and chemokines including TNF‐α, IL‐1β, IL‐6, IL‐8, MIP‐1α, IL‐12, IL‐18, as well as cytotoxic factors such as the aforementioned ROS and RNS, insulin degrading enzyme (IDE), and NO [51, 204, 205]. All of these factors are known to be upregulated in the AD brain as a result of increased microgliosis and astrogliosis, which is thought to facilitate a positive feedback loop that drives neuroinflammation, astrogliotic and microgliotic activity, Aβ deposition and tau hyperphosphorylation, and the overall progression of AD [51]. Given that dysregulated circadian rhythm has been shown to impair immune function and heighten neuroinflammation through the means explained thus far in this article, it is reasonable to assume that circadian dysregulation plays a very involved role in the heightened cerebral immune response that facilitates further neuroinflammation and increased Aβ deposition and tau hyperphosphorylation. It is possible that functional restoration to a dysregulated circadian rhythm could downregulate the activity of microglia and astroglia, which could aid in the treatment of AD by quelling neuroinflammation and restoring circadian‐mediated immune responses to normal function.

9 Therapeutic interventions

Unsurprisingly, the bulk of study in the realm of therapeutics pertaining to the restoration of optimal circadian function have been centered around the administration of exogenous melatonin. This administration of melatonin, usually in the form of a pill with doses ranging anywhere from 1 mg to 10 mg per pill, has thus far yielded inconclusive results. Melatonin administration generally occurs in the evening, with the aim of inducing a phase advance, meaning that the goal of the melatonin administration is to trigger the sleep period earlier in the evening than it would otherwise occur [206]. One study showed improvements in performance on cognitive tests as a result of evening time melatonin administration over a short period [207]. Other studies have shown that the administration of exogenous melatonin one hour before the ideal sleep onset time is effective for reducing nighttime movement and activity, likely meaning that it is effectively reducing connectivity in the DMN [10, 208]. One study involving the administration of 1 mg dose of melatonin at bedtime over the course of four weeks led to an increased rate of self‐reported “restedness” among the participants [209], while another study involving the administration of 5 mg doses of melatonin at bedtime led to increased sleep evaluation scores and a reduction in depressive symptoms over the course of two months [210]. Interestingly, a study involving the administration of a substrate containing melatonin (10 mg) and tryptophan (190 mg), increased cognitive test performances in a sample of eleven MCI patients over the course of twelve weeks, indicating that, at least in the short term, the restoration of dysregulated sleep–wake patterns in the preclinical stages of AD can yield cognitive restoration effects [211, 212].

It should be noted that the small sample size of a majority of these studies is a shared limiting factor. However, the success of sleep–wake cycle restoration effects upon the proper and consistent administration of exogenous melatonin makes melatonin therapy a promising avenue for the treatment of basic circadian dysregulation. To our knowledge, no studies have yet shown that consistent melatonin administration over time in MCI and preclinical AD populations has prevented or delayed the onset or progression of AD symptoms. This is likely due to the fact that while circadian dysregulation very obviously plays an important role in the preclinical and early stages of the disease, AD is a multifactorial ailment, and restoration of dysregulated sleep– wake cycles are not sufficient to rectify the physiopathological alterations induced by impaired circadian functionality and other factors known to contribute to AD pathology in the long term. In addition, it is necessary to mention that many studies involving melatonin based treatments in diverse samples, have yielded no significant effect. These findings are also limited by small sample size, however because of these findings, exogenous melatonin treatment can only be labeled as a promising, yet inconclusive avenue for the treatment of circadian dysregulation.

Other pharmacological approaches include the usage of benzodiazepines as a treatment for sleep phase disorders caused by rapid environmental changes. These therapeutics have been used as a common pharmacological therapy for the treatment of insomnia, however their prescription is increasingly limited due to their high rate of abuse. Triazolam, Zopiclone, and Zolpidem have shown effects at speeding circadian adaptation as a result of rapid environment changes and jet lag [213, 214]. Administration of these benzodiazepines have shown an increased sleep duration, sped up sleep–wake cycle adaptation, and increased sleep evaluation scores when administered after long flights through different time zones [215]. Temazepam on the other hand has shown no significant effects when administered after long flights through different time zones [215]. A melatonin receptor agonist, Ramelteon, has shown effectiveness in restoring a dysregulated circadian rhythm when administered in small doses at bedtime for 4 weeks [215]. Pycnogenol has also been shown to reduce sleep deprivation‐related neuroinflammation when administered in large doses several times per day [213, 215]. These findings indicate that benzodiazepine treatment is effective for the restoration of a dysregulated circadian rhythm, however much like the administration of exogenous melatonin, these treatments have shown no significant effect in the inhibition or delaying of AD onset and progression [215].

SSRIs and antidepressants have received limited study for their effects on restoring a dysregulated circadian rhythm [216]. Although they are not approved for this purpose, they are often prescribed to younger individuals who suffer from insomnia, under the assumption that they are effective at regulating sleep patterns. While many antidepressant drugs are effective for regulating circadian rhythms in depressed individuals, it is likely that this is an indirect consequence of the drugs’ treatment effects on serotonergic signaling and other physiological symptoms that underlie depression [158, 216]. Therefore, while these drugs may indirectly aid in the circadian rhythm restoration of a depressed individual, whether or not they are helpful for circadian regulation in the rest of the population with irregular sleep–wake cycles remains unclear. A review by Everitt and colleagues assessed twenty three different studies which examined the relationship between antidepressant drugs and circadian regulation and their effects on insomnia [217]. They found insufficient evidence to justify the prescription of antidepressants to patients with impaired circadian cycles, however they note that these studies were severely limited by the small number of participants and poor experimental methodology [217]. More study is needed to elucidate the relationship between circadian rhythm and antidepressant prescription, but it appears that these drugs have no noteworthy effect on sleep–wake cycle regulation.

That said, SSRIs specifically have yielded noteworthy, but also inconclusive results in terms of their relationship to AD pathology. Brendel and colleagues assessed individuals who showed Aβ deposition indicative of MCI and preclinical AD, and treated these subjects with SSRIs over a two year period [218]. A two year follow up examination revealed that the amount of gray matter degeneration in the frontal lobe was less in the subjects who were treated with the SSRIs than those who were not [218]. In addition, there was a small, yet noteworthy reduction in Aβ deposition in the SSRI group compared to the control group [218]. Other studies have shown that CSF levels of hyperphosphorylated tau and Aβ were entirely unaffected by long term treatment with SSRIs [219, 220]. Still, more studies have shown that long term treatment with SSRIs including Seraline, Fluoxetine, Citalopram, and Escitalopram facilitates a decreased rate of Aβ accumulation and oligomerization into the senile plaques which demarcate the severity of AD [218]. As can be seen, the extent to which SSRIs affect AD pathology remains inconclusive, however certain findings suggest that its further study is a worthwhile endeavor.

Non‐pharmacological photic therapies, specifically timed bright light exposure, have also received attention for their potential in restoring a dysregulated circadian rhythm. One study on 189 nursing home residents, found that daily treatment with bright light (1000 lux) or dim light (300 lux) in conjunction with the daily administration of exogenous melatonin (2.5 mg) over the course of 1.0–3.5 years slowed gray matter deterioration in an MCI sample, and attenuated symptoms of depression [173]. In addition, the melatonin administration was effective in restoring the sleep phase cycle to normal periodicity [173]. This combined treatment is thus far the only treatment modality for a dysregulated circadian rhythm that has also shown the ability to slow the progression of AD related pathology. For this reason, bright light therapy deserves to be studied intensively as a therapeutic avenue of great potential. However, there is nuance to this treatment that is not yet fully understood, as different brightness and exposure durations have yielded varying results.

One study involving bright light exposure to nursing home residents at greater than 2500 lux for one hour, showed no changes in sleep quality or duration, while also showing no change in the number of nighttime awakenings [221]. Other studies have shown that brighter light exposure (>3000 lux) has an effect on reducing the number of daytime naps, yet has no effect on sleep [4, 222]. Because most studies pertaining to bright light therapy in the context of circadian dysregulation and AD have thus far involved small sample sizes of nursing home residents, the field of study could benefit from a large scale study of MCI and other preclinical AD patients from diverse backgrounds to more firmly establish the efficacy of bright light therapy and determine the optimal levels of light brightness and exposure durations for best results.

10 Conclusion

Taken together, the findings elaborated on above implicate circadian dysregulation in the pathophysiology of AD. While certain details and the extent of its involvement remain poorly understood, it appears that circadian dysregulation could play a major role in the onset of MCI and preclinical AD symptoms, as well as playing a role in facilitating the transition of MCI into clinical AD.

Physiological findings have presented an apparent outline of circadian dysregulation and the manner in which it facilitates AD onset and progression, however the details and finer points of this process need to be elucidated through further experimentation. In sum, an age related decrease in endogenous melatonin and increases in cortisol synthesis and secretion, may lead to an increase in nighttime arousal. This heightened arousal during the ideal sleep period leads to increased DMN neuron connectivity, which in turn facilitates the downregulation of MT1 melatonin receptors in the SCN. This hormonal dysregulation could then induce adverse phase shifts that exacerbate the issue and catalyze further insomnia, while also leading to a decrease in nighttime glymphatic flow, CSF and ISF immune fluids, and lymphocyte abundance, as well as increases in the amyloidogenic cleavage of APP, Aβ deposition, and possibly tau hyperphosphorylation. These pathological alterations, along with the increased activation and abundance of microglia and astroglia, will lead to a state in which the neuroinflammatory and senile plaque burden is progressively exacerbated as circadian dysregulation worsens. In turn, neuroinflammation will progress as microglia continue their secretion of proinflammatory chemokines and cytokines and astroglia respond to the ever‐increasing quantity of neuronal lesions. This process, taken as a whole, could drive the progression of AD pathology over time, leading to MCI, and later clinical AD.

It is important to once again note, that the onset and progression of AD is multifactorial, and while circadian dysregulation plays a large part in driving Aβ deposition, neuroinflammation, and possibly tau hyperphosphorylation, it is not the sole driver and treatment of dysregulated circadian cycles will not inhibit AD pathology in the long term. This assertion is substantiated by the previously mentioned therapeutic trials which have shown that even when circadian rhythm is restored to normal in MCI and AD patients, at best, it slows the progression of AD and restores minor cognitive functions to these patients in the short‐term. These treatments have no effect on remedying AD‐typical symptoms throughout the brain in the long‐term.

Unless monotherapeutics are developed for the treatment of AD in the future, bright light therapy in combination with exogenous melatonin administration should be further explored as a component of polytherapeutic treatment for AD symptoms. The combination of bright light therapy and melatonin administration has proven more effective than any other therapeutics, and more study should be done to pin down their optimal doses and administration timing and duration.