We get busy sleeping, the brain gets busy cleaning (Image: Frank van Delft/Cultura/The Image Bank/Getty)

A good night’s sleep may literally clear the mind. The brain’s molecular waste-disposal system – discovered last year – is most active when we are slumbering, a study in mice suggests.

This adds to our understanding of the purpose of sleep and could inspire treatments for Alzheimer’s, Parkinson’s, and other neurological diseases linked to protein build-up in the brain.

“If this waste-disposal system is under the body’s intrinsic regulation, there could be a pharmacological way to turn the system up or down,” says Jeffrey Iliff at the University of Rochester Medical Center in New York, one of the study authors. “Perhaps this is a biological pathway that could be co-opted to rescue us from or improve neurological degeneration.”


Double the plumbing

Most of the body’s waste disposal is done by the lymphatic system, which clears damaged or misplaced molecules and pathogens from tissue and transports them to the blood, where they are broken down by the liver and excreted. But the traffic across delicate brain tissue is regulated by the blood-brain barrier, which does not allow the lymph to clean up in the way it does elsewhere.

Last year, Iliff and his colleagues discovered that the brain flushes out its waste via what he describes as a “second set of pipes”. This glymphatic system is made out of glial cells – sometimes called the “glue of the nervous system” because one of their primary functions is to help hold neurons in place. The glial cells form tubes around blood vessels in the brain. Special fluid flows in the space between the glial cells and the blood vessel and sweeps protein build-ups away from brain tissue, where they can cause disease.

The initial discovery of the system last year involved tracking the fluid flow using a powerful imaging technique called two-photon microscopy. Now the team has employed that same technique to compare glymphatic flow in conscious and sleeping mice.

They found that waste-clearing fluid flows much more rapidly in the brains of mice that were either sleeping or anaesthetised compared with awake mice.

Pipe dreams

The reason for the increased flow, the researchers suggest, could be that the glial cells shrink away from the blood vessels during sleep. This leads to a 60 per cent increase in volume between the outer glymphatic pipes and the inner blood vessels, letting more fluid pass through the brain.

Further testing showed that beta amyloid protein – which is associated with Alzheimer’s when it clumps together in the brain to form plaques – was cleared twice as rapidly from the brains of sleeping mice compared with mice that were kept awake.

“This is very important because I don’t think people have been thinking of how the sleep-wake cycle might influence a process of neurodegeneration,” says David Holtzman, a neuroscientist and Alzheimer’s researcher at Washington University in St Louis, Missouri.

“This work emphasises how we might want to think about ways our normal habits, even sleep, predispose us to certain problems later in life,” he says.

Journal reference: Science, DOI: 10.1126/science.1241224