It's very confusing.
This morning I woke up fully energized at 5:45AM and went to sleep at around 1:30AM. But I'll probably be really tired later and sleep an additional 6 hours in a nap. I've just screwed up my sleep schedule that much, and I'm fairly surprised; I didn't know the human body enjoyed a sleep break this much, but apparently it does because I was adjusted to it only after one night w/ an alarm clock (and now I take about an 8 hour break between two long naps in the AM, one from one-to-five-ish and 10-to-two-ish).
Normally a full night's rest of 8-9 hours will let me feel full energized the following day.