Though considered a waste of time by some, sleep is essential for the health and wellbeing of humans. Over the years, researchers have found sleeping allows the body to repair and restore cells, get rid of irrelevant memories, and even help kids grow taller and obtain better grades. Now, it appears that snoozing for an average of 8 hours daily enables the brain to cleanse itself and get rid of harmful toxins.
The latest research follows up on a 2013 study, which found a dramatic increase in the flow of cerebrospinal fluid (CSF) in the brains of rats and baboons during sleep. The fluid helped wash away harmful waste proteins accumulated in their brain cells during the day. The process acts a bit "like a dishwasher," said Dr. Maiken Nedergard, a neuroscientist at the University of Rochester who led the 2013 effort.
Since one of the waste products removed from the mammals' brain was beta-amyloid, a substance that forms a sticky plaque associated with Alzheimer's, Nedergard wondered if his findings could offer a new understanding of the progressive disorder, which causes human brain cells to degenerate and die. Though the idea was certainly plausible, further research needed to be conducted to verify that human brains "cleanse" in a similar fashion.
A team of scientists led by Dr. Laura Lewis, an assistant professor of biomedical engineering at Boston University, recently set out to do just that. To observe the brain during sleep, the researchers asked the study's 13 participants to eschew their comfortable beds and instead spend the night sleeping inside an MRI machine. Each person was also required to wear an EEG (electroencephalography) cap. The MRI machines allowed the scientists to measure the brain's blood oxygen and cerebrospinal fluid levels, while the EEG caps enabled them to monitor its electrical currents. "We had a sense each of these metrics was important, but how they change during sleep and how they relate to each other during sleep was uncharted territory for us," Dr. Lewis remarked. To emulate a realistic sleep cycle, the tests were conducted at midnight, and the subjects were asked to stay up late the night before to ensure they would drift into a deep sleep during the process.
The scientists, who published the results of the study in the journal Science in late 2019, found that similar to rats and baboons, large waves of cerebrospinal fluid washed through the participants' brains during non-REM sleep, the deepest part of the cycle. Even more interesting, the EEG readings showed that during this time, neurons started to turn off and on in a synchronized fashion. "There would be this electrical wave where all the neurons would go quiet," Dr. Lewis said. The halt in neuron activity caused less blood to flow into the brain, creating more room for the "cleansing" cerebrospinal fluid. The researchers are not sure how brain waves, blood flow, and CSF coordinate so perfectly. They speculate that as the blood leaves, pressure in the brain drops, and CSF quickly flows in to maintain a safe level of pressure.
Since patients with Alzheimer's have been observed to have less frequent and weaker brain waves, the researchers suspect that poor sleep in patients with neurological disorders may be impacting the brain's cleansing process and causing toxins to accumulate, eventually leading to degeneration.
"So we might expect that there are also fewer and smaller waves of cerebrospinal fluid in those disorders, and that might have an impact on how waste products are cleared," Dr. Lewis says. "We're running new studies to test how these CSF waves may change in healthy aging and in neurological disorders. We're also going to test whether this would be associated with less waste removal from the brain during sleep in these patients."
William Jagust, a professor of public health and neuroscience at the University of California, Berkeley, who was not involved in the study, believes the findings also suggest that people can reduce their risk of brain disorders like Alzheimer's by ensuring they prioritize high-quality sleep. So be sure to get your zzzs!
Resources: NPR.org, Wired.com, www.bu.edu