Some scientists argue that the purpose of sleep may not be restorative. In fact, they argue that the very question “why do we sleep?” is mistaken, and that the real question should be “why are we awake?”. If you are safe and warm and fed, it is a waste of energy to be awake and moving around (and possibly getting into trouble). Far better, this argument goes, is to be awake only when you have to and sleep when it suits you.
I like that. Although I don’t see how it would explain why we need to sleep.