Nedergaard and her colleagues showed that the mouse glymphatic system—which exchanges cleansing cerebrospinal fluid for protein-laden interstitial fluid in the brain—enlarges during sleep or anesthesia, and shrinks during awake periods. As an example of sleep's cleaning power, the group demonstrated that β-amyloid in the interstitial space disappeared faster while animals were sleeping.
Sleep researchers say the finding makes sense. It “fits with a long-standing view that sleep is for recovery—that something is paid back or cleaned out,” David Dinges of the University of Pennsylvania told Science Now. “It's not surprising, our whole physiology is changing during sleep,” Raphaelle Winsky-Sommerer, a lecturer in sleep at Surrey University, told the BBC. “The novelty is the role of the interstitial space, but I think it's an added piece of the puzzle, not the whole mechanism.”
Nedergaard told the BBC that the brain has limited resources, and can't devote energy to both the functions of awake states and house cleaning. “You can think of it like having a house party,” she said. “You can either entertain the guests or clean up the house, but you can't really do both at the same time.”