We evolved according to the natural rhythms of darkness and light; our bodily functions reflect this and undergo similar fluctuations. They perform best when we live in accordance, as much as possible, with these cycles.
In an editorial about sleep violence, a man chronicles what it's like to commit violent acts in his sleep (granted, his didn't go much further than punching the headboard or acting like a defensive lineman).
I still get amused by people who think sleep is a state of nothingness. Sleep is a break from busy wakefulness for the body's, but there's a lot going on in the brain making sleep far from a state of inactivity.