Deep sleep clears the brain of waste

A new study from Northwestern University describes how sleep clears the brain of waste, which is critical for cognitive health. The researchers examined brain activity in fruit flies and found that deep sleep restores the brain by clearing harmful proteins that may cause neurodegenerative disease.

“Waste clearance could be important, in general, for maintaining brain health or for preventing neurodegenerative disease,” said study senior author Dr. Ravi Allada. “Waste clearance may occur during wake and sleep but is substantially enhanced during deep sleep.”

The common fruit fly, Drosophila melanogaster, is a well-studied model organism for sleep, circadian rhythms, and neurodegenerative diseases. The neurons that control the sleep-wake cycles of fruit flies are strikingly similar to those found in humans. 

For the current investigation, Dr. Allada and his team examined a sleep stage in fruit flies that is similar to deep, slow-wave sleep in humans known as proboscis extension sleep (PES). During this stage of deep sleep, fruit flies repeatedly extend and retract their proboscis, or snout.

“This pumping motion moves fluids possibly to the fly version of the kidneys,” explained Dr. Allada. “Our study shows that this facilitates waste clearance and aids in injury recovery.”

The team manipulated the flies so that they could not achieve deep sleep. As a result, the flies struggled to clear an injected non-metabolizable dye from their systems, and were found to be more susceptible to traumatic injuries.

According to Dr. Allada, the study brings us closer to understanding the mystery of why all organisms need sleep. All animals are incredibly vulnerable when they sleep, particularly those in the wild. However, a growing collection of evidence shows that the benefits of sleep outweigh the increased risks. 

“Our finding that deep sleep serves a role in waste clearance in the fruit fly indicates that waste clearance is an evolutionary conserved core function of sleep,” wrote the researchers. “This suggests that waste clearance may have been a function of sleep in the common ancestor of flies and humans.”

The study is published in the journal Science Advances.

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By Chrissy Sexton, Earth.com Staff Writer