The brain’s activity during rest likely serves some essential function. The evidence for this importance starts with sleep’s ubiquity. All animals apparently sleep even though being unconscious and unresponsive greatly raises the risk of becoming another creature’s lunch. Birds do it, bees do it, iguanas and cockroaches do it, even fruit flies do it, as we and others demonstrated more than a decade ago.

Sleep must serve some vital function because all animals do it. Evidence suggests that sleep weakens the connections among nerve cells, which is a surprising effect, considering that strengthening of those connections during wakefulness supports learning and memory. But by weakening synapses, sleep may keep brain cells from becoming oversaturated with daily experience and from consuming too much energy.