“Promoting changes in brain circuitry may well be the fundamental purpose of slumber—the answer we’ve long been searching for. But it could also turn out to be a byproduct of something more fundamental still. For instance, some of the genes that are especially active during sleep seem to play a role in rebuilding cell membranes and synthesizing basic molecules like proteins and lipids. So sleep’s ultimate function could be to repair or rebuild brain cells in light of new experience, with reinforced circuitry as one result.” (Slate)