The purpose of sleep? To forget.

In two recent Science papers de Vivo et al. and Diering et al. probe the nightlife of the synapses that control the signalling between cells in our brain. They find substantial alterations in the structure and molecular machinery of synapses during sleep, providing strong evidence for synaptic downscaling during sleep and upscaling during wake, as well as clues to the molecular mechanisms. The idea is that our brain synapses grow during the day, and our brain circuits get more noisy. During sleep our brains pare back the connections to enhance the signal to noise ratio, as we forget some of the things learned during the day. Here is a summary graphic taken from the review by Acsády and Harris: