The new study of mice found that by "taking out the trash" during sleep, the brain clears away toxins thought to be responsible for brain disorders such as Alzheimer's disease.
"This study shows that the brain has different functional states when asleep and when awake," the study's lead author, Dr. Maiken Nedergaard, co-director of the University of Rochester Medical Center for Translational Neuromedicine, said in a university news release. "In fact, the restorative nature of sleep appears to be the result of the active clearance of the by-products of neural activity that accumulate during wakefulness."
The lymphatic system disposes cellular waste throughout the rest of the body, but this waste removal system does not include the brain, Nedergaard's team found in earlier research.
With the help of new imaging technologies, the researchers were able to examine a living mouse brain to determine exactly how the brain gets rid of waste. They discovered the brain has its own unique process, known as the glymphatic system, that is guarded by a complex gateway known as the blood-brain barrier.
Using the brain's blood vessels, the glymphatic system pumps cerebral spinal fluid through the brain's tissue, flushing waste into the circulatory system until it eventually reaches the liver.
Noting that the amount of energy used by the brain increases during sleep, the researchers suggested this is when the brain's waste-removal system becomes more active. The brains of the mice studied were 10 times more active during sleep, the investigators found.
The researchers explained that pumping cerebral spinal fluid requires a lot of energy and this process may only be possible at night when the brain is not actively processing information. They also pointed out the sleeping brains were able to remove much more amyloid-beta, the plaque-building protein associated with Alzheimer's disease.
"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," explained Nedergaard. "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."
The study, published Oct. 17 in Science, also showed that during sleep, cells in the brain "shrink" or reduce in size by 60 percent. This creates more space between cells so that waste can be removed more effectively. The researchers speculated that noradrenaline, a hormone that is less active during sleep, could help control this process.
Without the brain's waste-removal system, toxic proteins, such as amyloid-beta, would accumulate in the brain. The researchers pointed out that nearly every degenerative brain disorder is linked to the buildup of cellular waste products.
"These findings have significant implications for treating 'dirty brain' disease like Alzheimer's," concluded Nedergaard. "Understanding precisely how and when the brain activates the glymphatic system and clears waste is a critical first step in efforts to potentially modulate this system and make it work more efficiently."
Scientists note, however, that results obtained in animal experiments do not necessarily apply to humans.
The U.S. National Institute of Neurological Disorders and Stroke has more about the human brain.