Why do we sleep? We spend a third of our lives doing so, and all known animals with a nervous system either sleep, or show some kind of related behaviour. But scientists still don't know what the point of it is.
There are plenty of theories. Some researchers argue that sleep has no specific function, but rather serves as evolution's way of keeping us inactive, to save energy and keep us safely tucked away at those times of day when there's not much point being awake. On this view, sleep is like hibernation in bears, or even autumn leaf fall in trees.