Sleep is crucial for overall health. Getting a good night’s sleep can help boost the immune system, prevent weight gain, increase productivity, and more. Now a new study from Northwestern University reaffirms the importance of getting a good night’s sleep.

By observing the brain activity and behavior of fruit flies, researchers found that deep sleep has a restorative power to clear waste from the brain. Their findings are published in the journal Science Advances in a paper titled, “A deep sleep stage in Drosophila with a functional role in waste clearance.”

“Sleep is a highly conserved state, suggesting that sleep’s benefits outweigh the increased vulnerability it brings,” the researchers wrote. “Yet, little is known about how sleep fulfills its functions. Here, we used video tracking in tethered flies to identify a discrete deep sleep stage in Drosophila, termed proboscis extension sleep, that is defined by repeated stereotyped proboscis extensions and retractions.”

“Waste clearance could be important, in general, for maintaining brain health or for preventing neurogenerative disease,” said Ravi Allada, MD, the Edward C. Stuntz distinguished professor in neuroscience, chair of the department of neurobiology in Northwestern’s Weinberg College of Arts and Sciences, and senior author of the study. “Waste clearance may occur during wake and sleep but is substantially enhanced during deep sleep.”

The brain waste removal system, known as the glymphatic system, works to prevent the buildup of toxic debris. Poor sleep has also been linked to the buildup of biological markers for Alzheimer’s disease in humans.

The researchers observed proboscis extension sleep (PES), a deep-sleep stage in fruit flies, which is similar to deep, slow-wave sleep in humans. The researchers discovered that, during this stage, fruit flies repeatedly extend and retract their proboscis (or snout).

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

When Allada’s team impaired flies’ deep sleep, the flies were less able to clear an injected non-metabolizable dye from their systems and were more susceptible to traumatic injuries. Although fruit flies seem very different from humans, the neurons that govern flies’ sleep-wake cycles are similar to our own, which is why they have become an ideal model organism for sleep, circadian rhythms, and neurodegenerative diseases.

Allada said this study brings us closer to understanding the mystery of why all organisms need sleep.

“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,” the paper’s co-authors wrote. “This suggests that waste clearance may have been a function of sleep in the common ancestor of flies and humans.”

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