Hypnic headache (HH) is a rare sleep-associated primary headache disorder, usually affecting aged people, first described by Raskin in 1988. The headache attacks, single or multiple in one night, occur exclusively during sleep and tend to present at a consistent time each night, sometimes during a dream. Compared to the original description, newly reported cases have expanded the clinical spectrum of the disorder to include unilateral forms (about 40%, half of which are side-locked), forms with a longer duration (up to 3 h) and cases with onset in juvenile/adult age. The male predominance found in Raskin's series has not been confirmed by subsequent observations. To date the reported F/M ratio is 1.7/1. Pain is of severe intensity in less then one-third of cases and mild-moderate in about two-thirds. The location of pain is fronto-temporal in over 40% of cases; headache is throbbing in 38% of cases, dull in 57% and stabbing in less than 5%. Nausea is reported in 19% of cases; photophobia, phonophobia or both are present in 6.8%. Mild autonomic signs (lacrimation, nasal congestion, ptosis) may rarely be present. In 2004, HH was included in Group 4 of the International Classification of Headache Disorders-II (Other primary headaches). Sufficient evidence, mainly from polysomnographic studies, indicates that HH is a primary rapid eye movement (REM) sleep-related headache disorder of chronobiological origin. Lithium, melatonin, indomethacin and caffeine at bedtime are among the most effective therapeutic options. The pathophysiology of HH is still unclear. Available data allow speculation that, in predisposed subjects, an age-related impairment of suprachiasmatic nucleus could cyclically activate a disnociceptive mechanism leading to both a sudden awakening and headache. The mechanism may be precipitated by neurophysiologic events such as the strong reduction of firing occurring in the dorsal raphe nucleus during a REM sleep phase.