One thing that is amazing, I think, is how you can wake up at the time you want without an alarm clock. I know I can, at least. If I want to awake at whatever time, so long as I'm not too tired, I'll stop sleeping at around that time. The body has some sort of biological mechanism for determining time.
Although there's an explanation for it.
http://en.wikipedia.org/wiki/Circadian_Rhythm
I just thought this was relevant, that's all.