Basak Gurbuz Derman / Getty images
During deep sleep, the brain appears to wash away waste products that increase the risk of Alzheimer’s disease.
Numerous new research studies suggest that this stage of sleep – when dreams are rare and the brain follows a slow, steady rhythm – may help reduce levels of beta-amyloid and tau, two hallmarks of the disease.
“There’s something about this deep sleep that helps protect you,” said Matthew Walker, a professor of neurology and psychology at the University of California, Berkeley.
The study comes after decades of observations linking poor sleep to long-term memory and thinking problems, Walker said. “We now learn that there is a significant link between sleep and dementia, especially Alzheimer’s disease.”
The strongest evidence includes deep sleep, he says. Then the body temperature drops and the brain begins to produce slow, rhythmic electrical waves.
So Walker and a team of scientists set out to answer the question: “Can I look at your future and can I estimate exactly how much beta-amyloid you will accumulate over the next two years, the next four years, the next six years, just based on sleep you tonight? ”
To find out, Walker’s team studied 32 70-year-olds who took part in a sleep study that looked for slow electric waves that signaled deep sleep. None of the participants had memory problems.
The researchers used brain scans to monitor beta-amyloid levels in each participant for up to six years. And the results published in the November 2 issue of the magazine Modern biology, showed that people who are less deep have more beta-amyloid.
“We currently have a specific sleep signature that seems to help us better understand where you can sit on the Alzheimer’s risk trajectory in the future,” Walker said.
Other studies have found that lack of deep sleep is associated with higher levels of tau, which forms toxic entanglements in the brain cells of people with Alzheimer’s disease.
Scientists have some ideas as to why deep sleep appears to be able to reduce both beta-amyloid and tau.
In 2013, a study of mice found that their brains turned on something like a dishwasher during sleep.
“So things like amyloid beta, which are involved in Alzheimer’s disease, seem to actually be removed faster from the brain when the animal is asleep, compared to their waking state,” said Laura Lewis, an assistant professor of biomedical research at Boston University.
In 2019, Lewis led a team that showed how this dishwasher works in humans.
“We learned that these waves of fluid flow into the brain during sleep,” she says. “And this was happening on a much larger and slower scale than anything we had seen during wakefulness.”
Moreover, each wave of liquid is preceded by a large, slow electric wave.
So now scientists are looking for ways to cause the slow waves that signal deep sleep. Lewis says it’s easy – in rodents.
“There is a specific deep brain structure that, if stimulated, can cause sleep-like slow waves in the brain,” she says.
In humans, there is some evidence that rhythmic sounds can increase slow waves.
It is also possible to amplify slow waves by treating certain sleep disorders, says Dr. Yo-El Joo, an associate professor of neurology at the University of Washington in St. Louis.
Ju was part of a study of people with obstructive sleep apnea that repeatedly blocked the airways. These patients “seem to have a change in their ability to clear proteins or waste products from their brains,” she says. “And people with sleep apnea are at higher risk for online dementia.”
So Ju’s team looked for what would happen after the patients were successfully treated for apnea. The researchers found that the treatment resulted in deeper sleep and more beta-amyloid cleared from the brain.
And Ju says there was another effect: participants’ brains began to make less beta-amyloid.
“So I don’t know if sleep increases clearance or sleep reduces waste production,” she says.
Either way, sleep is important for brain health, Ju says, although she admitted she was a little sleep deprived the day we talked.
“My 2-year-old decided to sleep in my bed and eat a tortilla and a banana at 2 at night,” she says. “But I usually get pretty good sleep.”