Sleep is not an optional enterprise. All mammals do it. So do birds, reptiles, and even fruit flies. Rats deprived of sleep apparently die faster than those deprived of food. Sleep deprivation is a ruthlessly effective means of torture, as the new movie The Lives of Others shows in a stomach-turning scene. Yet the bedrock question—what purpose does sleep serve for us and the rest of the animal kingdom—remains oddly unsettled.
no one knows exactly what the underlying need is.
This Slate article discusses the theory that sleep plays a role in memory consolidation.