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October 18, 2013

Sleep: The ultimate brainwasher?

Every night since humans first evolved, we have made what might be considered a baffling, dangerous mistake. Despite the once-prevalent threat of being eaten by predators, and the loss of valuable time for gathering food, accumulating wealth, or having sex, we go to sleep. Scientists have long speculated and argued about why we devote roughly a third of our lives to sleep, but with little concrete data to support any particular theory. Now, new evidence has refreshed a long-held hypothesis: During sleep, the brain cleans itself.

Most physiologists agree that sleep has come to serve many different purposes, ranging from memory consolidation to the regulation of metabolism and the immune system. While the "core" purposes of biological functions such as breathing and eating are easy to understand, however, scientists have never agreed on any such original purpose for sleeping. The new study, by Maiken Nedergaard and colleagues at the University of Rochester in New York, provides what Charles Czeisler, a sleep researcher at Harvard Medical School in Boston, calls the "first direct experimental evidence at the molecular level" for what could be sleep's basic purpose: It clears the brain of toxic metabolic byproducts.

The new work, published online Thursday in Science, "fits with a long-standing view that sleep is for recovery — that something is paid back or cleaned out," says David Dinges, a sleep researcher at the University of Pennsylvania. It builds on Nedergaard's recent discovery, described last summer in Science Translational Medicine, of a network of microscopic, fluid-filled channels that clears toxins from the brain, much as the lymphatic system clears out metabolic waste products from the rest of the body. Instead of carrying lymph, this system transports waste-laden cerebrospinal fluid (CSF). Before the discovery of this "glymphatic system," as Nedergaard has dubbed it, the brain's only known method for disposing of cellular trash was to break down and recycle it within individual cells, she says.

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