HAMILTON: Either way, Ju says sleep is important to brain health, though on the day we spoke, she hadn't gotten that much. JU: So I don't know whether it's that sleep increases clearance or whether sleep decreases the production of waste products. And Ju says there was another effect - participants' brains began making less beta amyloid. The result - more deep sleep and more beta amyloid cleared from the brain. HAMILTON: So Ju's team looked to see what happened after treatment allowed better sleep. And people with obstructive sleep apnea are at higher risk for dementia down the line. YO-EL JU: They seem to have a change in their ability to clear proteins or, you know, waste products from their brain. Yo-El Ju was part of a team that studied patients with sleep apnea. Louis showed that treating a sleep disorder also helped. And researchers at Washington University in St. HAMILTON: There's some evidence that rhythmic sounds can increase slow waves in people. LEWIS: There's a specific deep brain structure that if you stimulate it, you can cause these sleeplike slow waves in the brain. So now scientists are looking for ways to induce the slow waves that signal deep sleep. HAMILTON: What's more, each wave of fluid was preceded by a large, slow electrical wave. LEWIS: We realized that there's these waves of fluid flowing into the brain during sleep. HAMILTON: In 2019, Lewis led a team that showed how this dishwasher works in people. LAURA LEWIS: 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, a biomedical engineer at Boston University, says that appears to clear out waste products. In 2013, a study of mice found that their brains switched on a sort of dishwasher during sleep. HAMILTON: Scientists also have some ideas about why deep sleep can reduce amyloid. WALKER: We have a specific sleep signature right now that seems to help us better understand where you may sit on the Alzheimer's disease risk trajectory in the future. And Walker says the results, published early this month in the journal Current Biology, showed people who got less deep sleep had more beta amyloid. The scientists used brain scans to track levels of beta amyloid in each participant for up to six years. HAMILTON: To find out, Walker and a team studied 32 older adults who had taken part in a sleep study. WALKER: Can I look into your future and can I accurately estimate how much beta amyloid you're going to accumulate over the next two years, the next four years, the next six years, simply on the basis of your sleep tonight? HAMILTON: Amyloid or beta amyloid is a substance that forms sticky clumps in the brains of people who are likely to develop Alzheimer's. WALKER: There is something about this deep sleep that is helping protect you against amyloid buildup in the brain. That's a time when dreams are rare, body temperature drops and the brain produces slow, rhythmic electrical waves. HAMILTON: Walker says the strongest evidence involves deep sleep. MATTHEW WALKER: We are now learning that there is a significant relationship between sleep and dementia, particularly Alzheimer's disease. Matthew Walker at the University of California, Berkeley, says that's no coincidence. JON HAMILTON, BYLINE: There's growing evidence that people who don't sleep well are more likely to develop Alzheimer's. NPR's Jon Hamilton brings us this report done with the NPR science podcast Short Wave. We continue to learn about the connection between sleep and Alzheimer's disease.