Alarm clock headache: A relatively rare form of headache in which the patient is awakened from sleep at the same time every night, usually between 1 and 3 o'clock, 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. The disorder almost always affects people over 65.

The precise cause of alarm clock headaches is unknown but there is evidence that the disorder 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. In the medical literature, the condition is called hypnic headache.