A new study implies that the reason animals all sleep has to do with mitochondria.

The mitochondria need a break from oxygen build up.^[ej]

Given that animals need to rest, it also makes sense for other maintenance tasks (like memory compression) to happen in that time, too.

When you think about it, it's wild that nearly every animal does something like sleep, since it's inherently dangerous.

But clearly the reason for it is something that all animals share.