The fundamental role of sleep might be to flush the brain of more than just unwanted thoughts, scientists have concluded.
Despite decades of effort, and clear evidence that a lack of sleep is fatal for flies, mice or humans, the role of the process that dominates at least a third of our lives has remained unclear.
Now University of Rochester researcher Maiken Nedergard and her colleagues, writing in Science, have found evidence that at least part of the role of sleep might be to cleanse the brain of a toxic build-up of metabolic rubbish that accumulates during the day.
Working with mice, the team injected dyes into the cerebrospinal fluid that bathes the brain and compared where the dye went when the animal was asleep versus when it was awake.
In sleeping animals, they found, the dye rapidly infiltrated throughout the brain tissue, indicating a rapid flux of the cerebrospinal fluid around the brain cells.
But in animals that were awake when the dye was administered, the spread was greatly reduced. The same result was achieved when sleep was induced with anaesthetic agents.
Next, the team looked at molecules like beta-amyloid, the protein which accumulates pathologically in the brains of patients with Alzheimer's Disease.
By continuously collecting small samples of the cerebrospinal fluid surrounding brain cells, they found beta-amyloid proteins were cleared twice as fast from the brains of animals when they slept.
The effect appears to be driven by activity in nerve cells that produce the nerve transmitter chemical noradrenaline and signal sleep or awakes states. This appears to open up the flow of cerebrospinal fluid by allowing the volume of the space around brain cells to increase by up to 60%.
Thus, the team say, "the restorative function of sleep may be due to the switching of the brain into a functional state that facilitates the clearance of degradation products of neural activity that accumulate during wakefulness..."