Hypnic headache: A relatively rare form of headache disorder in which the patient is awakened from sleep nightly, usually between 1 and 3am, with intense dull or throbbing pain over the whole head. Each episode may last up to 1 hour and be associated with nausea. Similar episodes may occasionally strike in the daytime. Hypnic headache almost always affects people over 65. The precise cause is unknown but there is evidence that hypnic headache is related to REM sleep. The first treatment option is lithium. If it is not effective or is not tolerated, indomethacin, flunarizine, and caffeine may be useful. Also called alarm clock headache.