This sounds like something that happens to me if I got no sleep the night before, or only like an hour or two.

I'm so tired I feel almost feverish the next night - and I'll be ready to go to sleep really early. But when I lay down I start to fall asleep and then, like you said, suddenly I wake up. And it's weird - I feel kind of panicky or something. I call that part burning off the sleep fever.

Then yeah, it takes a while before I can fall asleep again. But when I do , this time I sleep normally.

I suspect it's pretty common for people who are in sleep deprivation from the night before. Although I think I'm more sensitive to it than a lot of people - my friend for instance doesn't seem to be bothered by it at all.

When we were teenagers we used to sleep over at each others houses sometimes and stay up all night, and around about 4 am or so I'd always start feeling a little nauseous and get back pains and maybe a headache (just slight), and it wouldn't bother him at all. So I guess some people are just more sensitive to this kind of stuff and some aren't.