http://physics.aps.org/story/v21/st1 Why we sleep remains a mystery. Competing theories claim various “house-cleaning” brain activities occur during sleep, but they can’t say why we need to power down to accomplish them.
http://ngm.nationalgeographic.com/2010/05/sleep/max-text From birth, we spend a third of our lives asleep. After decades of research, we’re still not sure why. .... after 50 years of research, about the reason we sleep. “As far as I know,” he answered, “the only reason we need to sleep that is really, really solid is because we get sleepy.”