luckygehrig wrote:davidmarver is exactly right with the cycles. They've proven the existence of these cycles by measuring brainwaves. Basically you want to try to wake up in a period of "light" sleep rather than in the middle of REM. That's when you feel really tired no matter how much sleep you get.
I third that. From my experience, I feel a little more tired than usual after a deep sleep when I'm deeply entrenched in a dream state. I always go out for a run a few minutes after waking up and even though I'm feeling quite perky during the jog, I feel like I'm lagging a bit. I'm the type of person that can draw up energy even if I've gotten 3-4 hours of sleep. When I was small, I was always used to waking up early and training myself to do that and so that's part of it too I believe.
Naturally, there are times where eventually you'll feel more weary than tired though if you don't get the recommended hours of sleep a night and you can't necessarily nail it down on awakening from REM sleep. When I travel from US to Europe, I often find myself not sleeping for quite a long while (I adjust slowly to time zone variances more than usual) and there was a time I went 72 straight hours without sleep, even when I wanted to doze off. This is more of a special case, but it's worth putting out there in the open.