Some of the deepest mysteries in science have always involved the most mundane phenomenon. Is the earth moving or still? What is the sun? These two have been solved, but a third—Why do we sleep?—remains unsolved.
We spend a third of our lives asleep, but science still doesn't know what the function of this rest time actually is. We do know that when people don't sleep for extended periods, or get too little sleep, they develop a host of problems, such as impaired learning and decision making. Even memory takes a hit. And the longest any human has been recorded staying awake is about 11 days, a record set by a 17-year-old student at a science fair. Experiments keeping rats awake for more than two weeks always end in death. But why?
Now researchers have found what may be the beginning of an answer. "We show that the brain cleans itself during sleep," says Dr. Maiken Nedergaard, the author of the study, performed at Rochestor Medical Center. The experiment focused on the flow of glymphatic fluids in the spaces between the brain cells of mice. These fluids act something like a sewage system, draining out the waste products of the brain's activity. Nedergaard found that the space between brain cells increased 60 percent in mice that were asleep, versus those who were awake. This allowed the glymphatic fluid to flow much more freely, and remove the waste products from the brain much more effectively during sleep.
"The brain only has limited energy at its disposal, and it appears that it must [choose] between two different functional states — awake and aware, or asleep and cleaning up," Nedergaard said in a statement. "You can think of it like having a house party. You can either entertain the guests, or clean up the house, but you can't really do both at the same time."
This "cleaning out the brain" model not only may be a big step forward in understanding why we sleep, it also has important implications for diseases of the brain, such as Alzheimer's, which seem to be the result of toxic waste products building up in the brain.