Most weekends i wake up with a headache, especially if i sleep in..i don't understand this madness. sometimes it happens if i don't sleep in too.

Alcohol, No caffeine: If you drink alcohol the night before, it can cause a hangover. If you are used to caffeine through the weekdays, not having any on the weekends can cause withdrawl headaches. Finally, if it's neither case, then the sleep schedule disruption is the cause. Your body is craving consistency. The more you can stick to the same wake/sleep cycle every day, the better!