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 . . . 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.These "channels" are actually networks of glial cells, which pass the waste-laden CSF from one cell to the next and eventually out of the brain. This takes a lot of energy, and Nedergaard and her colleagues suspected that the cells might not be able to do their cleanup duties very well while also helping us think. Enter Lulu Xie, a graduate student who spent two years training mice to fall asleep in a microscope that can image the flow of CSF through the brain. (What do you do for a living? Oh, I train mice to fall asleep in microscopes.)
The new work, published online today 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 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.
By tracking the movements of dye throughout the brain, the team found that large amounts of CSF flowed into the brain during sleep, but not during the awake state, Nedergaard says. A comparison of the volume of space between nerve cells while the mice were awake and asleep revealed that the glial channels carrying CSF expanded by 60% when the mice were asleep. The team also injected labeled β amyloid proteins into the brains of sleeping mice and awake mice and found that during sleep, CSF cleared away this "dirt" outside of the cells twice as quickly—"like a dishwasher," Nedergaard says.Certainly an amazing piece of research. I find it very interesting that the better our science of sleep gets, the more it confirms the ancient view that sleep "knits up the raveled sleeve of care."