Abstract
Headache and sleep show so many similarities and clinical links that it is a little surprising to see growing research on this topic only in the last few years. On the other hand, basic and clinical research has now produced so many fascinating insights into the link of headache and sleep in such a short time that we can present a special issue of Cephalalgia contributed to this research.
Already in antiquity, the role of sleep for headache was recognized; good sleep was suggested to be a cure for headache, and bad sleep was said to be a trigger for headache. This link, the influence of sleep on headache occurrence, has been the major focus for centuries. In 1873, Edward Living said “the most frequent termination (for an acute attack of migraine) by far is sleep” (1), and Moritz Romberg suggested in 1853 that “the (migraine) attack is generally closed by a profound and refreshing sleep” (2). In the last decades, however, the general role of sleep on pain perception and the common underlying physiological and anatomic brain mechanisms both for headache and sleep have been more and more recognized. Basic research has meanwhile detected several morphological and physiological structures and functions that could explain some of the links mentioned above. In particular, the findings on the function and morphology of the hypothalamus both in headache and sleep disorders are pointing to common underlying pathways for trigeminal pain perception and sleep induction. In summary, it is more likely that headache and sleep disturbances share common underlying pathophysiological mechanisms than that both conditions just influence each other on a clinical level.
Interestingly, sleep disorders have been classified in a standardized system with our headache classification as an example. Headache and sleep are those fields in neurology with the most advanced classification and criteria systems. In the third edition of the International Classification of Headache Disorders (3), sleep is mentioned 38 times, and headache is mentioned in the International Classification of Sleep Disorders (ICSD) (4) as well with hypnic headache as a typical overlap syndrome.
However, the obvious links between headache or pain and sleep have become more and more part of headache and pain research rather than of sleep research. From a systematic point of view, we can differentiate the following aspects:
Underlying disorders leading both to headache and sleep disturbances: This includes neurodegenerative disorders that can go along with headache and with sleep disturbances. For example, several subtypes of Parkinson’s syndrome show headache and insomnia or rapid eye movement (REM) sleep behavior disorder (Schenck’s syndrome). The findings from basic research on the link between headache and sleep are described in the contribution by Holland. Headache disorders occurring exclusively or mainly during sleep: The most typical example for this link is hypnic headache. However, in several patients trigemino-autonomic cephalalgias (TAC) or migraine attacks also occur mainly during sleep. The two articles by Wang and Holle in this special issue highlight the pathophysiological and the clinical features of hypnic headache as a paradigmatic disorder for the link of sleep and headache. For the link between TAC and sleep, the article by Jensen presents the recent scientific findings. Sleep disorders or disturbances caused by headache: In this category, one can include insomnia as a result of headaches. 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). In this special issue, the article by Guidetti is focused on the specific situation in childhood. Sleep disorders directly causing headache: The most relevant example in this context is the morning headache in patients with sleep apnea syndrome because of hypercapnia during the night. Although this direct mechanism has not been shown unanimously, the epidemiological data show an obvious increase of morning headache in these patients. The contributions by Sand and by Russell in this special issue describe the epidemiological findings on this link. 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 (RLS) and for migraine and narcolepsy has been described. Another example that can also be included in category 4), however, is the occurrence of sleep apnea syndromes in cluster headache patients. We choose the example of RLS and migraine for this special issue; the article by Schürks describes this comorbidity in detail. The impact of headache medication on sleep and vice versa: Some drugs used in headache treatment can cause sleep disturbances. One well-known example is the occurrence of heavy dreams and sometimes even nightmares in beta-blocker treatment. On the other hand, 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 patient is suspicious for medication-overuse headache that can be worsened by central-acting drugs. The contribution by Nesbitt and Peatfield in this special issue is focused on this link.
In summary, all these aspects of the link between headache and sleep are covered in this special issue of Cephalalgia, and all authors are grateful to the editorial board and the editor-in-chief that they accepted this issue dedicated to their scientific interests in this field.
In addition, we were even able to include three original papers. This is very uncommon for a special issue with invited reviews. The contribution by Frese gives new data on the exploding head syndrome, an entity that has not yet been recognized by headache research but is included in the ICSD. The contribution by Westergaard presents a large epidemiological study from Denmark on the clinical co-occurrence of headache and sleep disorders. Finally, new data and observations on sleep apnea syndrome in cluster headache are described in a third paper.
May this special issue contribute to a better understanding of the link between headache and sleep disorders since both conditions cause an extreme impairment of quality of life, in particular in those patients suffering from both conditions.