Many animals sleep, even invertebrates.
But it has long been unclear why. What is the advantage of a prolonged period of unconsciousness, when you can't eat and you might get eaten?
Some research on mice now indicates that sleep serves to clean the brain.
Scientists injected the brains of mice with beta-amyloid, a substance that builds up in Alzheimer's disease, and followed its movement. They determined that it was removed faster from the brains of sleeping mice than awake mice.
The team also noticed that brain cells tend to shrink during sleep, which widens the space between the cells. This allows waste to pass through that space more easily.
Though the work involved mouse brains, lead researcher Dr. Maiken Nedergaard said this plumbing system also exists in dogs and baboons, and it's logical to think that the human brain also clears away toxic substances. Nedergaard said the next step is to look for the process in human brains.