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Abstract

Life on Earth has adapted to the changing patterns of light and darkness caused by the Earth's rotation. Known as circadian rhythms, these 24 h biological rhythms are regulated through light exposure and coordinate a range of behaviors such as sleep-wake cycles, eating, activity levels, and more. However, artificial light at night can disrupt these rhythms, altering how the human internal clock regulates sleep, metabolism, and even mood. This paper discusses some common forms of circadian disruption, such as nighttime light pollution and shift work, and examines the associated health outcomes. Some policy insights for reducing circadian disruption include regulating shift work, updating public lighting, and incentivizing circadian-based technologies. Regulating artificial lighting can improve human health and wellbeing, protect wildlife, and will allow future generations to continue enjoying starry nights.

Keywords

circadian disruption shift work artificial light

Research Highlights

Exposure to electric light at night is pervasive in modern society and has important health implications due to the disruptive effects of mistimed light exposure on the circadian system.

The circadian system regulates endogenous 24 h rhythms in brain, metabolic, and immunological function.

Shift work, a common cause of disruption to the circadian system, is associated with several negative health conditions, including metabolic syndrome, cancer, and depression.

This paper explores policy insights that may lead to societal improvements in circadian health, including lighting ordinances, regulation of shift work, and extended social services.

Additionally, incentivizing the development of novel circadian-based technology and educating the public about circadian rhythms could improve health.

Social Media Statement

What do nurses, flight attendants, and college students have in common? These groups have fluctuating daily schedules that can disrupt their 24 h “circadian” clock, leading to health complications. Here are policy insights for improving circadian health in the modern world.

Circadian Rhythms

The sun and other stars have played a pivotal role throughout human history. Prior to the invention of GPS, humans relied on the night sky for navigation. Yet the night sky has changed dramatically during the past 150 years, due in large part to the Industrial Revolution and the advent of artificial lighting. Artificial lighting has led to an increase in skyglow, the brightening of the night sky, which decreases visibility of the stars. Star visibility has decreased substantially during the past ten years and continues to decrease; estimates suggest that a child born today will only be able to see half the currently visible stars by their 18^th birthday (Kyba et al., 2023). Although humans no longer depend on the stars to navigate, skyglow from light pollution also has direct effects on wildlife migratory patterns and human health.

Life on Earth has adapted to the 24-h day caused by the Earth's rotation on its axis. Circadian (circa = about; diem = day) rhythms are internal biological rhythms that dictate many aspects of life such as sleep-wake cycles, metabolism, reproduction, activity, and more. These 24-h rhythms are generated in mammals by a primary clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus at the base of the brain. Additionally, nearly every cell in the body has its own clock, allowing for further synchronicity via neural and humoral cues coordinated by the SCN (Astiz et al., 2019).

Circadian rhythms are approximately, but not precisely, 24 h and thus require input from the environment to maintain a 24-h period over time. Daily light exposure is the main cue that fine-tunes humans’ 24-h rhythms. Special non-image-forming cells in the retina known as intrinsically photosensitive retinal ganglion cells (ipRGCs) detect changes in light and serve to entrain (synchronize) the SCN through light cues (Berson et al., 2002). ipRGCs contain melanopsin, a photosensitive pigment that is strongly activated by short wavelength visible (blue) light (∼480 nm) and is much less responsive to long wavelength visible light (∼600 nm) (Brainard et al., 2001). Daytime sunlight contains more blue wavelengths; as the sun approaches the horizon, shorter blue waves are scattered in the atmosphere and the longer, red waves reach the surface of the earth (Bedrosian & Nelson, 2017). Therefore, ipRGCs may have evolved to distinguish daytime from evening to better entrain circadian rhythms. ipRGCs are much slower to respond to light cues than other light sensitive cells in the eyes (e.g., rods, cones), meaning they are less responsive to acute changes in lighting and are more attuned to gross lighting changes across days and even months (Berson, 2003).

ipRGCs communicate ambient lighting information directly to the SCN through the retinohypothalmic tract. Along with entraining the SCN, information from the ipRGCs serves to regulate the production of melatonin, a hormone that is secreted in the absence of light. Melatonin aids in peripheral clock entrainment and is highly responsive to light; even dim light (<3 lux) is sufficient to suppress melatonin secretion in humans (Gooley et al., 2011). Without environmental cues such as lighting, circadian rhythms persist, albeit with a slightly longer than 24-h rhythmicity (Aschoff, 1965). Thus, light is a critical synchronizing cue that keeps the body's rhythms aligned with the environment. In the modern human environment, light exposure no longer follows the solar cycle. When individuals are exposed to light during their rest phase, they can display shifts in activity and melatonin secretion dependent on timing of light exposure (Khalsa et al., 2003). Although small phase shifts can be helpful in adjusting to changing seasons, abrupt or inappropriate timing of light exposure can have consequences for health and wellness.

Circadian Rhythm Disruptors in Modern Society

Before the industrial revolution, humans were exposed to minimal light at night. Technological advances have introduced new sources of light into our world including electronic screens (∼40 lux), lighting in homes (100–300 lux), and residential street lamps (15 lux) (Gaston et al., 2013). In contrast, the light from a full moon only emits 0.1–0.3 lux (Gaston et al., 2013). Indeed, 99% of Americans experience significant light pollution, defined as the alteration of natural nighttime lighting conditions (Falchi et al., 2016).

Artificial lighting affects human activity and health. Light emitting diode (LED) lighting, commonly used in industrial lighting and digital screens, often emits excessive blue light which is critical for circadian entrainment. However, blue-enriched lights negatively affect circadian rhythms of physiology and behavior. For example, having lights on in living spaces after dark delays the release of melatonin and may contribute to trouble sleeping (Gooley et al., 2011). Additionally, the wide prevalence of mobile phones with access to the Internet (i.e., smartphones), has led to light exposure throughout the day and night. Roughly 85% of US individuals own smartphones (Center, 2021), and many use their phones before bedtime (Hysing et al., 2015). The use of electronic screens delays the onset of sleep and decreases wakefulness the following day (Šmotek et al., 2020). Unfettered access to the Internet has also led many people, especially young people, to experience bedtime procrastination, the act of delaying sleep without external reasons (Kroese et al., 2014). People experiencing bedtime procrastination spend roughly 1 h more on their phone than their counterparts, leading to increased light exposure and impaired melatonin production (Kroese et al., 2014). Delaying sleep throughout the workweek leads many people to play sleep “catch up” on the weekends, a phenomenon known as “social jetlag”. Social jetlag arises from a misalignment between our body's natural sleep rhythms and social pressures (e.g., work and school) (Wittmann et al., 2006). Most work schedules do not allow for flexibility in start times, despite growing evidence that people show a range of preferences for the timing of activity. Known as chronotype (the sleep phase adopted by a person under natural conditions), these preferences often competition with the standard workday, leading people with later chronotypes to experience social jetlag (Roenneberg et al., 2003).

Perhaps the best example of chronic circadian disruption is night shiftwork, work done outside of the traditional nine-to-five schedule. About 25% of the adult US population performs night shift work, spanning a range of professions from nurses to truck drivers to factory workers (Kryger et al., 2010). The schedules of shift workers vary. Some maintain fixed schedules (shifts that begin at the same time of day), whereas others undergo rotating schedules (shifts that rotate start times according to a set schedule) (Safety, 2023). Rotating schedules can take many different forms, such as fast-rotating shifts that change working hours every 2 to 3 days with 2 days off every week, or longer schedules that rotate between day and night shifts every 1 to 2 weeks with 1 week breaks at the end of the month (Pacheco, 2023). Typically, individuals performing shift work will adjust to their nontraditional schedules and their rhythms will shift to match the time that they are awake (Andlauer et al., 1979). However, anywhere from 10% to 30% of individuals performing shift work are never able to adapt and instead develop shift work disorder, characterized by extreme sleepiness during work hours and insomnia during rest hours (Waage et al., 2009). Interestingly, individuals who have performed shift work for many years may lose tolerance (as shown by increased digestive issues, fatigue, and impaired sleep) (Andlauer et al., 1979). Therefore, even individuals adjusted to working during nontraditional workhours are prone to chronic light exposure and circadian rhythm disruption, which can have deleterious effects on health and wellbeing long term.

Health Consequences of Circadian Disruption

The health consequences of circadian disruption are vast, ranging from increased fatigue and lack of focus to cardiovascular diseases and certain cancers. One of the most common effects of disrupted circadian rhythms is altered sleep. Artificial light exposure, especially at night, impairs the ability to fall asleep and shortens sleep duration (Šmotek et al., 2020). Poor sleep quality is associated with diminished cognitive function (Harrison & Horne, 2000). Importantly, cognitive performance is time of day dependent (Schmidt et al., 2007), which may explain the increased rate of incidents during night shifts. Night shift workers are also at increased risk for chronic fatigue and sleep debt, which can increase the rate of accidents at work by 50%–100% (Wagstaff & Lie, 2011).

Disrupted circadian rhythms can affect not only cognition, but also impact mood and may lead to increased anxiety and depression. Depression, characterized by mood changes and diminished pleasure, is often accompanied by sleep disruptions (Belmaker & Agam, 2008), implicating circadian disruption as a key feature of depression and other mood disorders (Walker et al., 2020). Notably, it is not clear whether depression elicits circadian changes or whether circadian misalignment drives depressive symptoms. However, examples of chronic circadian rhythm disruption such as night shift work suggest that disrupted circadian rhythms contribute to increased risk for depression (Lee et al., 2017). Additionally, one small study reported that circadian misalignment predicted depression severity – the greater the disruption of circadian rhythms, the more severe the depressive symptoms (Emens et al., 2009). Therefore, circadian misalignment may account for the rising rates of depression globally.

Night shift workers display higher rates of metabolic syndrome, defined by a set of complex factors such as dysregulated glucose levels, increased blood pressure, and dyslipidemia (i.e., the imbalance of lipids such as cholesterol) (Sooriyaarachchi et al., 2022). Metabolic syndrome is related to an elevated risk for type II diabetes and cardiovascular diseases (Wilson et al., 2005). Accordingly, night shift work is also associated with increased risk for myocardial infarction and ischemic stroke, as well as coronary heart disease and hypertension (Chellappa et al., 2019). Night shift workers also have increased incidences of diabetes, especially in men (Gan et al., 2015).

Breast cancer also appears to be influenced by circadian rhythms and light exposure. In studies comparing women exposed to light at night to those in dark neighborhoods, light at night was found to be positively associated with breast cancer, but not other forms of cancer such as lung cancer (Kloog et al., 2010). Studies of women engaged in night shift work repeatedly report increased incidence of breast cancer in individuals working night shifts (Gehlert et al., 2020), with some evidence indicating that the number of consecutive night shifts may impact risk for breast cancer (Lie et al., 2011). Although it is unclear why circadian rhythm disruption is associated with breast cancer, one hypothesis points to the anti-tumoral properties of melatonin. Concurrently, women with breast cancer have lower melatonin concentrations in their urine compared to control individuals, although this effect appears modest and stage-dependent (Wang et al., 2014).

Finally, chronic circadian rhythms disruption such as night shift work can cause people to be on differing schedules from their family and friends who work traditional jobs, leading to decreased socialization and increased marital strife. Shift workers, especially those working slower rotations over the course of several weeks to months, report worsened marital quality and increased divorce rates than workers in traditional nine-to-five roles (White & Keith, 1990). Additionally, night shift workers report diminished ability to socialize due to their schedules and less time for leisure (Vitale et al., 2015), which may partially account for diminished quality of life amongst night shift workers (Kim et al., 2002). Night shift work can also impact families of shift workers. Children of night shift workers display increased behavioral problems and also show later bedtimes than peers of traditional workers, suggesting that night shift work may disrupt the circadian rhythms of both the worker and their families (Han, 2008; Lowson et al., 2013).

Policy Insights

Despite ample evidence of the health consequences of light pollution and chronic circadian misalignment, most people in developed nations are consistently exposed to artificial light at night. Eradicating all forms of circadian disruption is likely futile given the pervasiveness of digital screens and the reliance on shift workers in healthcare and infrastructure. However, several policy changes can mitigate the effects of light pollution on the environment, attenuate the potency of exposure to artificial light at night, and regulate shift work schedules to maximize worker health and well-being. Organizations should (a) implement lighting ordinances regulating outdoor lighting and neon signs, (b) develop and promote grants focused on updating the lighting in hospital wards and public schools, (c) improve regulations for night shift workers, (d) provide incentives for technological advancements considering light as a chronic disruptor of rhythmicity, (e) expand social service hours for night workers, and (f) educate the larger public on the consequences of circadian disruption.

Implement Lighting Ordinances Regulating Outdoor Lighting

Not all artificial light contributes to light pollution; rather, light pollution arises from excessive lighting in undesired locations. Thus, well-designed lighting features that control the spread of light could mitigate the effects of light pollution whilst maintaining night-time visibility. Unfettered artificial lighting can affect not only human health, but is also expensive and requires excessive energy, not to mention the devastating ecological consequences.

Street lighting at night is often used to increase visibility and deter crime. However, the relationship between lighting and crime is complicated. Although lighting often makes people feel safer, several reports show that lighting does not diminish crime and may in fact increase certain crimes such as vandalism (Sherman et al., 1996). Additionally, street lighting can increase glare (Kraus, 2016), leading to increased eye strain and visual impairment. Perhaps the largest drawback of outdoor lighting is the cost, as up to 30% of all outdoor lighting is considered wasted by unshielded lighting, leading to increased skyglow and over $3 billion in annual cost (International Dark-sky Association). To combat light pollution and cost, we recommend cities adopt lighting ordinances to regulate the amount and type of outdoor lighting allowed. To date, less than half of all US states have lighting ordinances, with the majority of ordinances regulating street lighting (Schultz, 2022). Streetlamps should be shielded so that the light is projected down to reduce light pollution while maintaining visibility. Blue-enriched lights, although beneficial during daylight, can disrupt melatonin secretion and should therefore be replaced with lights that preserve melatonin release at nighttime. Additionally, the timing of lighting should also be regulated to reduce impacts on wildlife and conserve energy. By adopting rigorous lighting ordinances, we can conserve not only energy and costs, but our wildlife and ecosystem.

Update Lighting in Hospital Wards and Public Schools

Hospitalized patients often experience poor sleep quality, with light being the number one contributing factor (Kulpatcharapong et al., 2020). Inadequate sleep is a contributing factor to disease severity and impaired recovery, and improving sleep is believed to improve patient outcomes. Notably, exposing patients to bright sunlight during daytime hours reduced depression, lessened pain responses, and even shortened stays compared to patients housed in dimmer rooms (Jamshidi et al., 2020). Additionally, the few studies examining the effects of lighting on hospital staff performance and satisfaction report that increased exposure to natural lighting was the highest contributor to increased satisfaction among staff (Mroczek et al., 2005). Therefore, exposing patients to bright, natural sunlight during typical waking hours and reducing (or eliminating) artificial light during rest hours improves outcomes. However, most hospitals rely on overhead LED lighting that keeps patient rooms at a standard 100–300 lux brightness, roughly 10 times dimmer than natural sunlight (10,000–25,000 lux), regardless of time of day.

Public schools follow similar lighting structures, despite evidence that students are more alert and may even perform better when exposed to more natural sunlight (Heschong & Mahone, 1999). Although updating current schools and hospitals with windows may not be feasible, we suggest diverting grants toward updating the lighting in schools and hospitals to LED lighting rather than fluorescent lighting. We also suggest adding dimmers to hospital rooms and automatic shades on all windows to help reduce light from outside during inappropriate hours. Federal and foundational grants should also fund further research on the effects of lighting such as LED on student performance, eye comfort, and patient recovery time as it is unclear how various forms of artificial lighting impact performance and recovery. Plans for future hospitals and classrooms should consider natural lighting in design. In settings where it is not possible to introduce more natural light, we suggest introducing programs where students and patients are allowed outside for brief periods of the day to help synchronize their circadian rhythms. Early hours of the morning are best to help entrain circadian rhythms and promote alertness and can be implemented relatively easily through already existing curriculum such as physical education. Providing greater access to natural lighting and reducing inappropriate timing of light exposure can increase recovery time in patients, improve student alertness and test scores, and boost overall morale amongst patients, students, and staff.

Regulating Night Shift Work Requirements

Night shift workers are perhaps the population most susceptible to disrupted circadian rhythms and the resultant health consequences. However, despite a growing body of research highlighting the implications of shift work on health and family life, there are limited regulations for shift workers. The Fair Labor and Standards Act (FLSA) does not require extra pay for shift workers, nor does it stipulate how shift work should be performed to minimize health risks. Few states have any additional laws defining or regulating shift work, allowing instead organizations to create their own shift schedules, resulting in a range of schedules for shift workers across the country. Regulating shift work schedules could not only minimize circadian disruption for workers but will also streamline research on shift workers to allow for more robust evidence on the health and safety effects of chronic shiftwork.

Several sources offer suggestions for regulating night shift work (see Rosa & Colligan, 1997). Here, several amendments to shift work are suggested to promote worker health and safety. First, appending the FSLA to include a section about shiftwork. This section advises requiring all organizations who employ shift workers to follow predictive scheduling, wherein employers are required to give at least 2 weeks’ notice about all shift changes. Predictive scheduling allows for greater employee planning and can help individuals avoid constant phase shifts that may affect their health. Also exposing night shift workers to bright light (∼5,000 lux) at the start of their shift can help entrain their rhythms and promote alertness (Boivin & Boudreau, 2014). This can be accomplished by increasing the brightness in break rooms or offices, although in hospitals care will need to be taken to ensure that these lights do not disturb patients. Additionally, employers should consider hiring occupational therapists to provide employee health assessments such as measuring sleepiness and fatigue. Understanding the rates of fatigue amongst shift workers can thus allow employers to provide improved care and services (e.g., “napping rooms” where workers can rest) and may allow for faster identification of individuals who are susceptible to shift work disorder. Finally, requiring organizations reliant on shift workers to provide paid-time off to ensure workers take time to readjust their schedules and catch up on sleep and family time. Federal and state policies detailing the requirements for shiftwork and providing benefits for employees are critical for protecting the large population of workers who are at increased risk for health and familial strife.

Incentivize Circadian-Based Technology

Technological advancements, such as the invention of digital screens and the Internet, have undoubtedly contributed to the rise in circadian misalignment in modern society. Although screens have revolutionized work and leisure lives, they also impact health and sleep. Thus, innovations aimed at reducing the effects of screens and other circadian disruptors should be considered a high priority for entrepreneurs. Enlisting entrepreneurship grants could motivate inventors interested in the circadian rhythm. For instance, blue light-blocking glasses and “night mode” on screens are innovations that improve eye comfort and reduce circadian disruption. Other inventions such as artificial lighting that more closely mimics the properties of natural lighting, for example, could have valuable implications on residential and public lighting. Technologies aimed at altering lighting parameters should account for a range of light throughout the day: adjusting both light intensity and blue wave content across the day to maintain circadian rhythmicity and health (Moore-Ede et al., 2023). These grants should prioritize entrepreneurs invested in designing technology to offset circadian disruption to improve human health and comfort.

Expand Social Services

As the population of night shift workers continues to grow and the United States becomes a “24-h society”, social services such as healthcare and counseling must also grow with them. Although shift workers are more susceptible to depression and anxiety, they often struggle to access services designed to assist. For instance, many physicians and counselors’ offices are only open 9–5, requiring shift workers to go during the day (when they should be sleeping) or go instead to 24-h emergency care which often has longer wait times. Night shift workers also struggle to find adequate childcare, as many nurseries and child services are only open during standard work hours.

Employers must therefore fill in gaps where employees struggle to find care. Policies should require employers to provide counseling and health services at accessible times. Tools such as telehealth are cost-effective ways to provide healthcare access for employees with nonstandard work hours and can be easily implemented through mobile phones. Frequent health screenings can help vulnerable populations such as shift workers identify risk factors and seek treatment without sacrificing rest hours. Finally, employers should provide childcare as an incentive for employees working nontraditional hours. Airports, such as in San Francisco and Phoenix, are building on-site childcare facilities, which will offer around-the-clock care for the children of employees (Lampert & Singh, 2023; Palcare). These sites offer tuition assistance if needed and allow parents the ability to create their own schedules every month. Offering childcare can alleviate employee stress and may be a useful incentive against larger e-commerce companies that are able to offer higher hourly wages.

Education

Finally, enlisting circadian researchers and occupational therapists to publish lay resources will help individuals to educate themselves on chronic circadian disruption. Just as the Center for Disease Control (CDC) and Sleep Foundation provide tips for improving sleep hygiene, researchers should provide sources where people can learn about circadian rhythms and circadian misalignment. Websites, online forums, and videos are perhaps the most easily accessible and can provide information and resources for individuals about how the circadian system functions and how improper lighting can alter people's intrinsic clocks. For example, the International Dark-Sky Association website (https://www.darksky.org) details resources related to the human and wildlife health consequences of light pollution. Popular podcasts such as The Daily (https://www.nytimes.com/column/the-daily) offer a platform for circadian researchers to discuss their work and the impact of light on our rhythms. Scientific educators such as CrashCourse (https://www.youtube.com/@crashcourse) provide short educational videos on YouTube about sleep and circadian rhythms. However, few websites and resources are dedicated solely to circadian rhythmicity, and many only address circadian systems as they relate to sleep without addressing the many other biological contributions of people's internal clocks. Notably, many of these sources are meant to supplement information from more formal venues such as the classroom. Therefore, integrating circadian lectures into high school and college biology courses could target younger populations who are most likely to be affected by bedtime procrastination and blue light from digital screens.

Conclusions

Disruption of circadian rhythms is an ever-present problem in a society under constant light. Reducing unnecessary light at night is critical for the health and well-being of humans and wildlife, as is regulating worker schedules to mitigate health risks and job dissatisfaction. Policymakers should prioritize reducing light pollution, especially in rural areas with large wildlife populations, and regulating shift workers’ schedules while providing social services for families of shift workers. Policymakers should also fund research exploring how various artificial lighting alters student performance and patient recovery, prioritizing natural lighting whenever possible. Undertaking these policy changes will require concerted efforts from hospital and school administration, local and federal government, researchers, innovators, and shift workers. If successful, however, then we may protect human and wildlife health for generations to come.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article

ORCID iD

Lourdes K. Davis

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Health Effects of Disrupted Circadian Rhythms by Artificial Light at Night

Abstract

Keywords

Research Highlights

Social Media Statement

Circadian Rhythms

Circadian Rhythm Disruptors in Modern Society

Health Consequences of Circadian Disruption

Policy Insights

Implement Lighting Ordinances Regulating Outdoor Lighting

Update Lighting in Hospital Wards and Public Schools

Regulating Night Shift Work Requirements

Incentivize Circadian-Based Technology

Expand Social Services

Education

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

ORCID iD

References