Hypnic headache has been described in several case reports since 1981 and is regarded as an idiopathic headache disorder. In this review of 71 cases in the literature, the clinical features, neurophysiologic including polysomnographic findings, and treatment procedures are analyzed and the pathophysiology of this condition, which remains however speculative, is discussed. There is some evidence that hypnic headache is related to REM sleep. The analysis shows that hypnic headache most probably is an entity among the idiopathic headache disorders unassociated with structural lesions and does not belong to the trigeminal-autonomic cephalalgias. Lithium shows the best efficacy; indomethacin, flunarizine, and caffeine may also be useful.