International Headache Society (IHS) code and diagnosis: 4.5 Hypnic headache

World Health Organization (WHO) Code: G44.80

Short description: Hypnic headache (HH) is a rare, recurrent, sleep-related, primary headache disorder. It usually begins after 50 years of age. The attack occurs at night when sleeping, waking the patient with a mild to moderate headache, severe in more than 20% of patients. Pain is bilateral in about two-thirds of cases. The attack usually lasts from 15 to 180 minutes, but longer durations have been described. Caffeine and lithium have been effective treatments in several reported cases. The pathophysiology of HH is unknown, but there is some evidence that it is related to rapid eye movement (REM) sleep.

Previously used terms: Hypnic headache syndrome, “alarm clock” headache

Raskin first described hypnic headache in 1988 (35). Since then, more than 90 cases were reported in the literature (1, 2, 3,5, 6, 7, 8, 9,11, 12, 13, 14,17, 18, 19,22, 23, 24, 25, 26, 27, 28,30, 31, 32, 33,35, 36, 37,40, 41, 42, 43, 44). In the largest case study HH was diagnosed in 0.07% of all headache patients assessed annually at a specialty clinic (7). Although this is an estimated approximation of the true prevalence, it reflects the rarity of this syndrome.

The exact pathophysiologic mechanisms of HH have not yet been elucidated. It has been postulated that HH may be the result of a chronobiologic disorder, serotonin and melatonin dysregulation, or a disturbance of REM sleep (5,8,9,17,31,33,37,38).

Raskin hypothesized that HH results from a disturbance of the mammalian biologic pacemaker of the brain residing within the suprachiasmatic nuclei (SCN) (34). Dysfunction of this “biologic clock,” which generates circadian rhythms, has been linked to other phasic disorders such as bipolar illness, cluster headaches, and jet lag (34). A disturbance of the regulating system of the SCN could account for the clocklike regularity of headaches in HH.