It's long been known that sleep, much like eating or breathing, is a vital process for humans and animals. Insufficient shut-eye in people impairs cognition, curbs energy levels, and has been linked to a bevy of illnesses. But while scientists have explored myriad mechanisms that might explain why sleep is so critical, they've yet to come up with a firm answer. Now, new research offers yet another compelling theory: sleep allows our brains to clean themselves up.

In a paper published in Science, a research team out of the University of Rochester Medical Center (URMC) built on previous work that had identified what's called the glymphatic system. Much like the lymphatic system clears unwanted toxins, byproducts, and other materials from the body, the glymphatic system — which URMC investigators discovered last year — conducts that same scrubbing in our gray matter. "We've known for a long time that the brain lacks a traditional lymphatic system," says study co-author Rashid Deane, a neurologist at URMC. "But the glymphatic system, we now know, eliminates waste buildup instead."

"Helping the brain eliminate unwanted byproducts."

This latest study sought to examine how the glymphatic system behaved during sleep. Using an imaging technique called two-photon microscopy, researchers observed the glymphatic system and noted that it was significantly more active when mice were asleep compared to when the animals were up and about. "What we found was about a tenfold increase in activity," Deane explains. "That tells us that sleep appears to be significant in helping the brain eliminate unwanted byproducts."

Intriguingly, the team also found that space between brain cells increased during resting states — allowing more room for the cleaning process that's key to the glymphatic system. More precisely, that extracellular space more readily allows cerebrospinal fluid (CSF) to flow along and pick up waste products. From there, waste is ushered out of the brain and eventually broken down in the liver.

"You can either entertain the guests or clean up the house."

Though the team doesn't yet know why the glymphatic system speeds up during sleep, they speculate that it has something to do with energy allocation. Because the process requires significant energy consumption, the brain might need to be much less active — expending less energy on waking activities — for cleaning to take place. "You can think of it like having a house party," says Maiken Nedergaard, the study's lead author. "You can either entertain the guests or clean up the house, but you can't do both at once." This cleaning process might also play a role in neurological disorders: the glymphatic system helps clear away amyloid-beta proteins, the study found. An accumulation of those proteins are associated with Alzheimer's disease. Intriguingly, so too is insufficient sleep, Deane notes.

"These mechanisms are something we need to keep teasing out."

Of course, the finding doesn't yet offer any definitive answer on why we sleep. It might even be one of several explanations. Other compelling theories include the idea that rest is vital for memory storage and consolidation, or the proposal that sleep allows the body to conduct a host of restorative processes including muscle growth and tissue repair. "These mechanisms are something we need to keep teasing out," Deane says. "This certainly may not be the only process that makes sleep so imperative, but I do think it is one of them."