It is one of life’s great enigmas: why do we sleep? Now we have the best evidence yet of what it is for – allowing housekeeping processes to take place that stop our brains becoming overloaded with new memories.

All animals studied so far have been found to sleep, but the reason for their slumber has eluded us. When lab rats are deprived of sleep, they die within a month, and when people go for a few days without sleeping, they start to hallucinate and may have epileptic seizures.

One idea is that sleep helps us consolidate new memories, as people do better in tests if they get a chance to rest after learning. We know that, while awake, fresh memories are recorded by reinforcing connections between brain cells, but the memory processes that take place while we sleep have remained unclear.

Support is growing for a theory that sleep evolved so that  connections in the brain can be pruned down during slumber, making room for fresh memories to form the next day. “Sleep is the price we pay for learning,” says Giulio Tononi of the University of Wisconsin-Madison, who developed the idea.