Deep sleep takes out the trash

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

By examining the brain activity and behavior of fruit flies, the researchers found that deep sleep has an ancient restorative power to clean up brain waste. These residues potentially include toxic proteins that can lead to neurodegenerative diseases.

“Waste disposal can be important, in general, to maintain brain health or to prevent neurogenerative diseases,” said Dr. Ravi Allada, senior author of the study. “Waste elimination can occur during waking and sleeping, but is substantially increased during deep sleep.”

The study will be published today in the journal Advances in Science.

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

Although fruit flies look very different from humans, the neurons that govern the fly’s sleep-wake cycles are remarkably 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 his team examined prolonged proboscis sleep (PES), a stage of deep sleep in fruit flies, which is similar to deep slow wave sleep in humans. The researchers found that during this stage, fruit flies repeatedly extend and retract their proboscis (or snout).

“This pumping motion moves the fluids, possibly to the fly kidney version,” said Allada. “Our study shows that this facilitates waste disposal and helps with injury recovery.”

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

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

“Our discovery that deep sleep plays a role in eliminating waste in the fruit fly indicates that waste elimination is a centrally conserved, evolutionary function of sleep,” write the article’s co-authors. “This suggests that waste disposal may have been a function of sleep in the common ancestor of flies and humans.”