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For centuries scientists and philosophers have debated what our brains do during sleep. Now, for the first time, researchers have solid evidence that a good night's sleep may literally clear the mind. In a study published in the journal Science, researchers found that the space surrounding brain cells—called the interstitial space—may increase during sleep, allowing the brain to flush out toxins that build up during waking hours. Previous research shows that proteins linked to neurodegenerative diseases, including beta-amyloid, build up in the interstitial space.
According to Dr. Spira, these findings provide a potential mechanism for the link between poor quality sleep and greater cognitive impairment. "It isn't a mechanism we imagined, but it's an intriguing and plausible explanation for what we saw," he says.
For the study, researchers injected dye into the cerebrospinal fluid (CSF) of mice and watched it flow through their brains while simultaneously monitoring electrical brain activity. (CSF is a clear fluid that bathes and cushions the brain and spinal cord; it is continuously produced and reabsorbed.) The dye flowed rapidly when the mice were asleep or anesthetized, but slowed to a sludge-like crawl when the mice were awake.
These same researchers also injected mice with labeled beta-amyloid and measured how long it lasted in their brains when they were awake or asleep. The results: interstitial space volume increased by 60 percent when the mice were asleep. In addition, toxic waste products, including beta-amyloid, disappeared at twice the rate in the brains of sleeping mice compared to those who were awake.
"The restorative function of sleep may be due to the switching of the brain into a state that facilitates the clearance of waste products that accumulate during wakefulness," says Rashid Deane, Ph.D., research professor in the department of neurosurgery's Center for Translational Neuromedicine at the University of Rochester Medical Center in New York. So perhaps there's not only less production of beta-amyloid during sleep, but more clearance as well.
Brain cells are highly sensitive to their environment. Toxins can interfere with nerve function and damage cells, so it's essential to quickly and efficiently remove waste products from the interstitial space. Yet, unlike every other organ in the body, the brain doesn't have a conventional lymphatic system to flush out waste products, explains Dr. Knopman. Instead, CSF recirculates through the brain, interchanging with interstitial fluid and removing toxic proteins, including beta-amyloid.
This plumbing system in the brain, which Dr. Deane's team has dubbed the glymphatic system, offers a potential solution to a mystery that has baffled brain researchers: how does the brain get rid of waste without the help of a lymphatic system?
"CSF is produced in the middle of the brain," explains Dr. Deane. "It flows over the brain along the blood vessels, percolates through the brain tissue between cells, and then finds its way out through the venous part of the vascular system." A bonus: in the flushing process, the brain may not only get rid of waste products but also may take up beneficial products.
When you host a party, you don't begin the process of cleaning up until after the guests head home. Brain cells operate similarly. When they're busy working and supporting normal function, they are not clearing material. During the sleep phase, they switch roles and get rid of waste products.
Previous research seems to support these findings. A 2009 study, also published in Science, reported that the amount of beta-amyloid found in the interstitial fluid of a mouse model of AD increased markedly during periods of sleep deprivation. And a 2012 study, published in the medical journal Archives of Neurology, showed that beta-amyloid levels in the spinal fluid of humans rose during waking hours and fell during sleep—a pattern that was more pronounced in healthy young people.
