Slipknot's internal clock answer was exactly what came into my head when I read your question, DMO. It sounds like that's exactly what's waking you up unless there's some other contributing reason, like a medicine you're taking, too much caffeine, a noisy sleeping environment, a wakeful or wiggly bed partner, or an overly full bladder.

I have the same thing. I often wake up after four and a half hours of sleep and can't go back to sleep--or can't for a couple of hours. It happens to me at least a couple of times a week, and the only thing I can think of it that it's my own body clock (circadian rhythm) going off at an earlier-than-convenient time. I've just given into it and normally I get up and study and drink coffee, then go on to school, then take a nap in the afternoons when I get home. Some folks believe melatonin supplements can help reset the sleep-wake patterns, but I don't know how to use/dose melatonin for that purpose and am not inclined to try it myself. Sedatives are so potentially risky that I don't think they're a good idea.

Slipknot definitely had the right idea, I think. Why don't I move this to our medical area, which seems like a better category, and let's see what other folks have to say?