A growing body of research suggests that deep sleep can reduce a person's risk for Alzheimer's disease. Here's what the latest research says, according to NPR's Jon Hamilton.
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About Alzheimer's disease
Alzheimer's disease is a type of dementia traditionally defined as a clinical syndrome involving the progressive decline of cognitive abilities—particularly memory loss—that ultimately results in the loss of independence. About 5.8 million Americans have Alzheimer's disease, and CDC projects that number will reach 13.9 million by 2060 as baby boomers age. There is no proven treatment for the disease, though there are medications that temporarily alleviate the disease's symptoms.
So far, research into potential treatments for Alzheimer's disease have mostly focused on the beta amyloid protein, which accumulates in the brains of patients with Alzheimer's years before they experience memory loss. Researchers have hypothesized that the beta-amyloid buildup in the brain is responsible for the cognitive decline seen in Alzheimer's patients.
Largely, researchers have agreed that Alzheimer's disease always progresses the same way, with amyloid accumulating in a patient's brain, then a protein called tau appearing, and a patient's neurons dying. That widely held belief suggests attacking amyloids is key to preventing the disease.
Research suggests deep sleep may lower levels of beta-amyloid, tau
Several studies have shown that a lack of deep sleep—which occurs when the brain falls into "a slow steady, beat," a person's body temperature drops, and dreams become rare—is associated with higher levels of beta-amyloid and tau, Hamilton writes for NPR's "Shots." For instance, a study published last year in the journal Science Translational Medicine found that people who did not get deep sleep had more tau.
But the opposite appears to occur when people get more hours of deep sleep, research shows. In fact, the latest evidence suggests deep sleep can help to lower a person's beta-amyloid and tau levels, Hamilton writes.
According to Hamilton, scientists have found some evidence that explains why deep sleep appears to help reduce a person's levels of tau and beta-amyloid.
In 2013, for example, researchers in a landmark study published in the journal Science found that, during sleep, the brains of mice activate a mechanism that functions like a dishwasher to scrub away waste products—including beta-amyloid, Hamilton writes. "So things like amyloid beta, which are implicated in Alzheimer's disease, seem to actually be removed more rapidly from the brain when an animal is asleep versus when they're awake," Laura Lewis, an assistant professor of biomedical research at Boston University, told Hamilton.
And in 2019, a separate study led by Lewis that was published in the journal Science showed that dishwasher mechanism also works in people. "We realized that there's these waves of fluid flowing into the brain during sleep," Lewis told Hamilton. "And it was happening at a much larger and slower scale than anything we'd seen during wakefulness."
According to the researchers involved in the 2019 study, a large, slow electrical wave preceded each wave of fluid. Lewis told Hamilton that the slow waves signal deep sleep and, if stimulated, they may help prevent Alzheimer's disease.
"There's a specific deep brain structure that if you stimulate it, you can cause these sleep-like slow waves in the brain," Lewis told Hamilton.
And now, a new study published in the Nov. 2 issue of the journal Current Biology adds to the evidence showing deep sleep may protect people against Alzheimer's disease, Hamilton writes.
For the latest study, Matthew Walker, a professor of neuroscience and psychology at the University of California-Berkeley, and a group of scientists sought to determine whether they could estimate future beta-amyloid levels based on people's levels of deep sleep. To do so, the scientists looked at the brain scans of 32 adults in their 70s who had no memory problems and had participated in a sleep study examining their electrical waves for up to six years to monitor their beta-amyloid levels.
According to the researchers, the participants' deep sleep levels could be used to predict their beta-amyloid levels. Overall, the researchers found that participants who got fewer hours of deep sleep had higher levels of beta-amyloid. The researchers concluded that their findings suggest "sleep may serve as a marker of future Alzheimer's disease risk and the speed of progression."
Scientists seek ways to induce deep sleep
The growing body of evidence on the connection between Alzheimer's disease and deep sleep is driving scientists to look for ways to induce certain aspects of deep sleep in patients, Hamilton writes.
For instance, scientists are examining whether they can stimulate the slow waves in the brain that occur during deep sleep, according to Hamilton. And so far, researchers have found some evidence showing rhythmic sounds can boost those slow waves in people's brains, Hamilton writes.
Yo-El Ju, an associate professor of neurology at Washington University in St. Louis, told Hamilton there's also evidence that treating sleep disorders can help to increase those slow waves. Ju and a group of researchers examined what happened in the brains of patients with obstructive sleep apnea after receiving treatment, and they found that treatment led to an increase in deep sleep, lower levels of beta-amyloid, and a reduction in the brain's production of beta-amyloid among the participants.
According to Yu, their findings mean it's unclear "whether it's that sleep increases clearance or whether sleep decreases the production of waste products." But regardless of the answer, Yu said sleep is vital to brain health (Hamilton, "Shots," NPR, 11/17; Winer et al., Current Biology, 9/3).