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 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, 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.

In the original work, Nedergaard’s group showed that glia, the brain's non-neuronal cells, control the flow of CSF through channels in their cell membranes. "If we delete the channels in glial cells, the flow almost stops," Nedergaard says. Because the transport of fluid across cell membranes requires a lot of energy, Nedergaard and her team had a hunch that the brain would not be able to both clean and process sensory information at the same time and decided to test whether the activity of the glymphatic system changed during sleep. Lulu Xie, the new study's first author, spent the next 2 years training mice to relax and fall asleep on a two-photon microscope, which can image the movement of dye through living tissue.

Once Xie was sure the mice were asleep, based on their EEG brain activity, she injected a green dye into their CSF through a catheterlike device in their necks. After half an hour, she awakened them by touching their tails and injected a red dye that the two-photon microscope could easily distinguish from the green. By tracking the movements of red and green 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. Such proteins can aggregate as pathogenic plaques inside cells and are associated with Alzheimer’s disease, she says.

Many neurological diseases—from Alzheimer's disease to stroke and dementia—are associated with sleep disturbances, Nedergaard notes. The study suggests that lack of sleep could have a causal role, by allowing the byproducts to build up and cause brain damage. "This could open a lot of debate for shift workers, who work during the nighttime,” Nedergaard predicts. "You probably develop damage if you don’t get your sleep."

One unknown, however, is whether the need to remove waste products actively regulates sleep—whether, for example, the buildup of metabolic byproducts makes us sleepy. Researchers also wonder how the fluid-filled channels change shape during sleep, and whether clearing waste actually improves the function of neurons.

Nor are other researchers certain that cleansing is sleep’s sole core function. No one function of sleep necessarily rules them all, says sleep scientist Derk-Jan Dijk of the University of Surrey in the United Kingdom. “Sleep probably has many functions,” he says, just as the weekend is variously for shopping, socializing, and cleaning the house.

But now that Nedergaard and her colleagues have identified this nightly brainwashing in mice, Czeisler says, scientists can investigate whether it occurs in all species, and to what extent. “One could imagine that different species have evolved different additional functions of sleep to suit their different habitats, … but this will help resolve if there is some shared function of sleep across species,” he concludes.