Sleep debt is probably a factor too. Good, restful sleep comes from long-term good sleeping habits (like jacobgrey mentioned). Actually, if you're sleep-deprived for a prolonged period of time, even if you "catch up" on the sleep by sleeping in, it wouldn't really help because of something called REM rebound, where you spend more time in REM sleep (where you dream and often of stressful things), leading to feeling more tired when you wake up.
The human body is fascinatingly vengeful. Like "if you don't take care of me I'll make you suffer"
