There is a cool blog/website called Barking up The Wrong Tree that recently talked about two sleep periods Ancient books talked about the notion of two sleeping periods. Further probing shows that in the not so distant past, two distinct periods of sleep were the norm.
Before modern electricity and artificial light, it seems are natural Circadian Rhythm is to sleep for two periods. The first is to go to sleep when it gets dark, followed by a period of restful awakeness for an hour, then fall asleep again till the sun comes up. If modern man is removed from all artificial lights, this again is the natural rhythm he will adapt.
It seems this hour was used for meditation, prayer, sex, reading and problem solving. Blood worked has shown that this hour is incredibly healthy and relaxing. It wasn't looked on as a bad thing like it is today. In fact the notion of the 8 hour block of sleep is relatively new. The New York Times had an article entitled "Rethinking Sleep," that delves into this much deeper.
I never had heard of this natural rhythm before. It's an interesting fact. The take away, turn the lights off earlier if you can and if you wake up in the middle of the night, don't fight it. Relax and enjoy it.