We spend around a third of our lives asleep, but this time-consuming activity is far from wasted time. A new study of fruit flies further highlights how deep sleep may play an invaluable role in cleansing nasty waste products from the brain, like a literal brainwash.
As reported in the journal Science Advances this week, scientists led by Northwestern University were studying sleepy fruit flies (Drosophila) and made a curious observation: when the flies drifted off to a state of deep sleep, they repeatedly extended and retracted their nose (or proboscis). The team believes the flexing of the nose is a pumping motion that helps to increase the flow of hemolymph (invertebrates’ equivalent of blood) around the body. Along with delivering nutrients, this flow of liquid also appears to flush out waste byproducts from the brain, like a nightly neural spring clean.
“Waste clearance could be important, in general, for maintaining brain health or for preventing neurodegenerative disease," Dr Ravi Allada, senior author from the Department of Neurobiology at Northwestern University, said in a statement. "Waste clearance may occur during wake and sleep but is substantially enhanced during deep sleep."
"This pumping motion moves fluids possibly to the fly version of the kidneys," added Allada. "Our study shows that this facilitates waste clearance and aids in injury recovery."
To find these conclusions, the team injected a bunch of sleep-deprived fruit flies with a special dye and kept tabs on how it moved around their body. They found that the sleep-deprived flies were significantly less capable of clearing the dye from their systems if they were deprived of sleep and unable to complete this nose movement. They were also less able to recover from body injuries, further hinting at the restorative power of sleep.
Fruit flies might seem like a strange choice of animal to study, but they are one of the most commonly used model organisms for biomedical science. Like humans, they also have a 24-hour cycle of rest and activity, making them a useful tool to understand how sleep works in humans. Even though we don’t have twitching noses nor a poppy seed-sized brain, the researchers suspect sleep plays a very similar waste clearance role in humans and waste clearance is an evolutionarily conserved core function of sleep.
Back in 2019, another study published in Science showed how the human brain is “cleansed” during sleep. When we enter certain stages of sleep, the volume of blood in the brain will decrease and a watery liquid called cerebrospinal fluid will flow in, flushing through your brain in pulsing waves. Much like this new study, the researchers speculated that the flow of cerebrospinal fluid helped to flush out toxic proteins from the brain.