I kinda figured this out in high school. I slept irregular hours throughout the week, in bed around 10:00 PM weeknights, but staying up until 1:00 - 2:00 AM on weekends and sleeping until 9:00 - 10:00. I had headaches 2-3 days week.<p>Then I got a job that required a 7:00 AM start on the weekends, and the late nights stopped. So did the headaches.