What does Alarm clock headache mean?

Alarm clock headache meaning in Medical Dictionary

a comparatively rare type of headache where patient is awakened from rest on top of that every evening, usually between 1 and 3 o'clock, with intense dull or throbbing discomfort within the entire head. Each event may last up to an hour and become related to sickness. Similar attacks may occasionally strike within the day. The disorder typically affects individuals over 65.