Brains Sweep Themselves Clean Of Toxins During Sleep   
   3:41   
       
   October 17, 2013 4:59 PM ET   
   Heard on All Things Considered   
   Jon Hamilton 2010   
       
   What do our brains do while we sleep?   
   Katherine Streeter for NPR   
       
   While the brain sleeps, it clears out harmful toxins, a process that may   
   reduce the risk of Alzheimer's, researchers say.   
       
   During sleep, the flow of cerebrospinal fluid in the brain increases   
   dramatically, washing away harmful waste proteins that build up between brain   
   cells during waking hours, a study of mice found.   
       
   "It's like a dishwasher," says Dr. Maiken Nedergaard, a professor of   
   neurosurgery at the University of Rochester and an author of the study in   
   Science.   
       
   The results appear to offer the best explanation yet of why animals and people   
   need sleep. If this proves to be true in humans as well, it could help explain   
   a mysterious association between sleep disorders and brain diseases, including   
   Alzheimer's.   
       
   Nedergaard and a team of scientists discovered the cleaning process while   
   studying the brains of sleeping mice.   
       
   The scientists noticed that during sleep, the system that circulates   
   cerebrospinal fluid through the brain and nervous system was "pumping fluid   
   into the brain and removing fluid from the brain in a very rapid pace,"   
   Nedergaard says.   
       
   The team discovered that this increased flow was possible in part because when   
   mice went to sleep, their brain cells actually shrank, making it easier for   
   fluid to circulate. When an animal woke up, the brain cells enlarged again and   
   the flow between cells slowed to a trickle. "It's almost like opening and   
   closing a faucet," Nedergaard says. "It's that dramatic."   
       
   Nedergaard's team, which is funded by the National Institute of Neurological   
   Disorders and Stroke, had previously shown that this fluid was carrying away   
   waste products that build up in the spaces between brain cells.   
       
   The process is important because what's getting washed away during sleep are   
   waste proteins that are toxic to brain cells, Nedergaard says. This could   
   explain why we don't think clearly after a sleepless night and why a prolonged   
   lack of sleep can actually kill an animal or a person, she says.   
       
   So why doesn't the brain do this sort of housekeeping all the time? Nedergaard   
   thinks it's because cleaning takes a lot of energy. "It's probably not   
   possible for the brain to both clean itself and at the same time [be] aware of   
   the surroundings and talk and move and so on," she says.   
       
   The brain-cleaning process has been observed in rats and baboons, but not yet   
   in humans, Nedergaard says. Even so, it could offer a new way of understanding   
   human brain diseases including Alzheimer's. That's because one of the waste   
   products removed from the brain during sleep is beta amyloid, the substance   
   that forms sticky plaques associated with the disease.   
       
   That's probably not a coincidence, Nedergaard says. "Isn't it interesting that   
   Alzheimer's and all other diseases associated with dementia, they are linked   
   to sleep disorders," she says.   
       
   Researchers who study Alzheimer's say Nedergaard's research could help explain   
   a number of recent findings related to sleep. One of these involves how sleep   
   affects levels of beta amyloid, says Randall Bateman, a professor of neurology   
   Washington University in St. Louis who wasn't involved in the study.   
       
   "Beta amyloid concentrations continue to increase while a person is awake,"   
   Bateman says. "And then after people go to sleep that concentration of beta   
   amyloid decreases. This report provides a beautiful mechanism by which this   
   may be happening."   
       
   The report also offers a tantalizing hint of a new approach to Alzheimer's   
   prevention, Bateman says. "It does raise the possibility that one might be   
   able to actually control sleep in a way to improve the clearance of beta   
   amyloid and help prevent amyloidosis that we think can lead to Alzheimer's   
   disease."   
       
       
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