Deep sleep takes out the trash

A new study from Northwestern University reaffirms the importance of getting a good night’s sleep.

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

“Cleaning up waste could be important, in general, for maintaining brain health or preventing neurogenerative diseases,” said Dr. Ravi Allada, lead author of the study. “Waste elimination can occur during awakening and asleep, but is dramatically improved during deep sleep.”

The study will be published today in the journal Scientific advances.

Allada is the Edward C. Stuntz Distinguished Professor of Neuroscience and Head of the Department of Neurobiology at the Northwestern Weinberg College of Arts and Sciences. He is also associate director of the Center for Sleep and Circadian Biology at Northwestern. Bart van Alphen, postdoctoral fellow in Allada’s lab, was the first author of the article.

Although fruit flies appear to be very different from humans, the neurons that govern flies’ sleep-wake cycles are surprisingly similar to ours. 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 her team looked at proboscis extension sleep (PES), a stage of deep sleep in fruit flies, which is similar to slow wave deep sleep in humans. Researchers have found that during this stage, fruit flies repeatedly extend and retract their proboscis (or snouts).

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

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

Allada said this study brings us closer to understanding the mystery of why all bodies 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 crucial waste elimination – outweigh this increased vulnerability.

“Our finding that deep sleep plays a role in waste disposal in the fruit fly indicates that waste disposal is an essential function of sleep conserved in evolution,” write the co-authors of the article. “This suggests that waste disposal may have been a function of sleep in the common ancestor of flies and humans.”