Biological clock could be one of a few factors and it doesn't just apply to sleep. All animals have a biological clock including us. Ever notice how you automatically wake up at the same time as your alarm if you don't set it? (That is if you didn't stay up all night or were on some drunken binge.)

http://www.crystalinks.com/biologicalclock.html

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.