Since it’s relevant: Polyphasic sleep.
I’m actually going to write an article for the forums when I get the chance. I’ll probably start work on it this weekend.
Like Mew said, I believe the cycle you’re in when you wake up greatly affects how you feel when you do. I’ve gotten up before with no more than four hours feeling superb, and yet there are nights when I get 8 and a half hours and wake up feeling miserable.