Every night, while we lie asleep, blind, dumb and almost paralyzed, our brains are hard at work. Neurons in the sleeping brain fire nearly as often as they do in a waking state, and they consume almost as much energy. What is the point of this unceasing activity at a time when we are supposedly resting? Why does the conscious mind disconnect so completely from the external environment while the brain keeps nattering on?

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.