Abstract
Headache and sleep obviously show several clinical links, and even in antiquity good sleep was thought to be a cure for headache and bad sleep was said to be a trigger for headache. In the paper by Lateef and co-workers (1), a particular aspect of these links has been investigated: the comorbidity of poor sleep, in particular different types of insomnia, with headache (separately for headache in general, migraine without aura, migraine with aura). The authors show that headache in general, but not specifically migraine without or with aura, is associated with an approximately twofold increased risk for insomnia. This confirms previous studies which also linked sleep disturbances to headache in general (2). The authors further speculate about the underlying reason for this association in three ways. It might be that subjects with headache have a pathophysiology leading to both headache and insomnia; it might be that sleep disturbances are a trigger for headache in general; and it might be that headache induces insomnia.
The strength of this study lies in the large representative population-based patient sample, which were interviewed in the United States with a focus on one specific sleep disturbance, namely insomnia. However, the interview questions for migraine and in particular for the migraine aura were not very specific, and it is doubtful whether the different headache diagnoses are completely reliable (e.g. some aura subtypes were not included). Furthermore, the study defined increased daytime sleepiness as one aspect of insomnia, wheres it is classified as hypersomnia in the International Classification of Sleep Disorders (ICSD) (3) and might not only be a result of insomnia in the night but a result of other sleep disorders.
The obvious links between headache and sleep have increasingly become part of headache rather than of sleep research. From a systematic point of view, we can differentiate the following aspects:
Underlying disorders leading to both headache and sleep disturbances: This includes neurodegenerative disorders which can go along with headache and with sleep disturbances. For example, several subtypes of Parkinson’s syndrome show headache and insomnia or a disorder called REM sleep behavior disorder (Schenck’s syndrome) (4). Headache disorders occurring exclusively or mainly during sleep: The most typical example for this link is hypnic headache (5). However, in several patients, trigemino-autonomic cephalalgias or migraine attacks have also been found to occur mainly during sleep (6). Sleep disorders or disturbances caused by headache: In this category, one can include insomnia as a result of headaches. It might be that most patients in the study by Lateef et al. (1) fall into this category. However, we should also be aware that many headache patients show a good sleep and that sleep can be just a good treatment for headache (very obvious in childhood migraine). Sleep disorders directly causing headache: The most relevant example in this context is the morning headache in patients with sleep apnea syndrome because of the hypercapnia during the night. Although this direct mechanism has not been shown unanimously (7), the epidemiological data show an obvious increase of morning headache in these patients. Comorbidity of sleep disorders (in particular parasomnias) and headache disorders: Some epidemiological studies suggest a comorbidity of specific sleep disorders and specific headache disorders. For example, a comorbidity for migraine and restless legs syndrome (8) and for migraine and narcolepsy (9) has been described. Another example, however, can also be included in category (iii) is the occurrence of sleep apnea syndromes in cluster headache patients. It is still undetermined whether there is a common underlying pathway leading to both apnea and cluster headache attacks or whether the nightly hypercapnia triggers cluster headache attacks (10). Impact of headache medication on sleep: Some drugs used in headache treatment can cause sleep disturbances. One well-known example is the occurrence of heavy dreams and sometimes even nightmares during beta blocker treatment. Impact of sleep medication on headache: Some drugs used in the treatment of sleep disorders, such as benzodiazepines in the treatment of insomnia, might induce or worsen headaches. This is particularly important when a headache patient has possible medication oversuse headache, which can be worsened by central acting drugs.
Meanwhile, basic research has detected several morphological and physiological structures and functions which could explain some of the links mentioned above (11). In particular, the findings on the function and morphology of the hypothalamus in both headache and sleep disorders point to common underlying pathways for trigeminal pain perception and sleep induction. Another important finding involves the specific functions of orexin and melatonin in sleep and in trigeminal pain perception. In summary, it is more likely that headache and sleep disturbances share common underlying pathophysiological mechanisms than that the conditions directly influence each other. A phenomenon worthy of further study is the link between REM sleep and pain perception, as we have well-documented examples for the respective disorders co-occurring on both sides, for example, hypnic headache and REM sleep behavior disorder.
However, the study by Lateef and co-workers provides an example of the problems and restrictions in clinical research on sleep and headache. On one hand, we need large epidemiological studies to identify the burden of the comorbidity of headache and sleep. These studies are, however, necessarily biased by insecure diagnoses on both sides. Thus, besides large-scale epidemiological studies, we need well-designed experimental studies which adequately identify specific sleep disorders and specific headache disorders and then look at the pathophysiological link in animal models. Both headache and sleep disorders have one big advantage in comparison to many other neurological disorders. For both fields, we have worldwide accepted, both comprehensive and detailed, modern classification systems allowing standardized research (3,12). We also have animal models for many aspects both of headache pathophysiology and of sleep physiology. It is our responsibility to seek cooperation with sleep researchers. Headache researchers interested in the link between headache and sleep should at least be familiar with the systematic of sleep disorders and/or with the established models of experimental sleep research.
In the future, we will hopefully be able to better understand the link between headache and sleep disorders, as both conditions cause an extreme impairment of quality of life, in particular in those patients suffering from both conditions.