Deep sleep takes out the trash

A new study from Northwestern University confirms the importance of a good night’s rest.

By examining the brain activity and behavior of fruit flies, the researchers found that deep sleep has an ancient, restorative power to clear waste from the brain. This waste may contain toxic proteins that can lead to neurodegenerative diseases.

“Waste disposal can generally be important to maintain the health of the brain or to prevent neurogenerative diseases,” said Dr. Ravi Allada, senior author of the study, said. “Waste disposal can occur during wakefulness and sleep, but is significantly improved during deep sleep.”

The study is published in the journal today Scientific progress.

Allada is the Edward C. Stuntz Distinguished Professor of Neuroscience and Chair of the Department of Neurobiology at the North-West Weinberg College of Arts and Sciences. He is also co-director of Northwestern’s Center for Sleep and Circadian Biology. Bart van Alphen, a postdoctoral fellow in Allada’s laboratory, was the first author of the newspaper.

Although fruit flies look very different from humans, the neurons that control flies’ sleep-wake cycles are strikingly similar to our own. For this reason, fruit flies have become a well-studied model organism for sleep, circadian rhythms, and neurodegenerative diseases.

In the current study, Allada and his team examined the pendulum swing (PES), a deep sleep stage in fruit flies, similar to the deep, slow wave sleep in humans. The researchers discovered that fruit flies repeatedly pull and pull their string (or snout) during this stage.

“This pumping motion moves fluids to the flight version of the kidneys,” Allada said. “Our study shows that it facilitates waste disposal and helps repair injuries.”

When Allada’s team impaired the sleep of flies, the flies were less able to remove an injected, non-metabolizable dye from their systems, and were more susceptible to traumatic injuries.

Allada said this study brings us closer to understanding the mystery of why all organisms need sleep. All animals – especially those in the wild – are incredibly vulnerable when they sleep. But research is increasingly showing that the benefits of sleep – including important waste disposal – outweigh this increased vulnerability.

“Our finding that deep sleep plays a role in waste disposal in the fruit fly suggests that waste disposal is an evolutionarily preserved core function of sleep,” the co-authors of the newspaper wrote. “This suggests that waste disposal could have been a function of sleep in the common ancestor of flies and humans.”