For a long time, researchers weren’t quite sure why we sleep. There are a number of different theories. Some are evolutionary: For instance, sleep keeps us out of trouble at night and away from animals who wake up and hunt after the sun goes down. Some are physiological: Sleep lets us conserve precious energy. And some are anecdotal: Parents joke that they’d go nuts if sleep didn’t exist, since it affords them a much-needed break from their kids.