A new study has found a strong association between slow-wave sleep loss and an increased risk of developing dementia in older adults. Individuals over 60 years old are 27 percent more likely to develop dementia if they lose just 1 percent of slow-wave sleep each year, according to the study. Slow-wave sleep, or deep sleep, is the most restful stage of sleep, during which brain waves and heart rate slow, and blood pressure drops. It is also known as the third stage of a human 90-minute sleep cycle, lasting about 20 to 40 minutes.
Researchers examined 346 participants from the Framingham Heart Study who had completed two overnight sleep studies between 1995 and 1998 and between 2001 and 2003, with an average of five years between testing periods. The participants had no record of dementia at the time of the 2001-2003 study and were over 60 years old in 2020. By comparing participants' first and second sleep studies, researchers discovered a link between each percentage point decrease in slow-wave sleep per year and a 27 percent increased risk of developing dementia. Over the 17 years of follow-up, 52 dementia cases were recorded among the participants.
"Slow-wave sleep, or deep sleep, supports the aging brain in many ways, and we know that sleep augments the clearance of metabolic waste from the brain, including facilitating the clearance of proteins that aggregate in Alzheimer's disease," said Matthew Pase. "However, to date we have been unsure of the role of slow-wave sleep in the development of dementia. Our findings suggest that slow-wave sleep loss may be a modifiable dementia risk factor."
The risk of developing dementia increased to 32 percent when focusing specifically on Alzheimer's disease, the most common form of dementia, which affected 6.9 million Americans in 2022 and is projected to affect nearly 14 million by 2060. The study noted that overall, the rate of slow-wave sleep was found to decrease from age 60 onward, with this loss peaking between the ages of 75 and 80 and then leveling off after that. Low levels of slow-wave sleep were linked to a higher risk of cardiovascular disease, taking medications that can impact sleep, and having the APOE ε4 gene, which is associated with Alzheimer's disease. Lack of sleep is also directly associated with a higher risk of developing conditions such as obesity, type 2 diabetes, and other cardiovascular diseases.
Quality sleep plays an important role in the process of cleansing the brain, supporting the aging brain by augmenting the clearance of metabolic waste, including toxic proteins like beta-amyloids associated with Alzheimer's disease. During sleep, the fluid surrounding brain cells removes molecular garbage, including amyloid protein, which accumulates in sticky clumps called plaques and is believed to play a key role in Alzheimer's disease. "The brain produces amyloid during the day," and "the longer a person is awake, the more amyloid accumulates, and the less time the brain has to eliminate it." The liver and kidneys filter the garbage from the bloodstream, and the body expels it.
"One of the first places where tau accumulation appears is areas of the brainstem important for regulating sleep and wakefulness," explained Joe Winer, postdoctoral fellow in neurology and neurological sciences at the Stanford Center for Sleep and Circadian Sciences. "So we believe that the appearance of tau in these areas very soon is going to alter the sleep-wake cycles of people."
Experts note that sleep disorders, especially sleep apnea, are also associated with a higher risk of dementia. Sleep apnea tends to occur in people who are overweight or have diabetes, conditions that are also related to dementia. "Even when the effect of these other problems is removed, sleep apnea seems to confer its own independent risk of dementia," said Diego Carvalho, assistant professor of neurology at the Mayo Clinic Sleep Medicine Center. "This may be because sleep apnea limits the amount of oxygen that reaches the brain, which can increase brain inflammation and damage blood vessels and cells."
Sleeping too much also seems to be related to a higher risk of dementia, although perhaps in a more indirect way. If a person usually stays in bed for more than nine hours a night or takes several naps during the day, it may be a sign that they are sleeping very poorly. The need to sleep excessively may be related to a mental or physical disability. Mental disorders, such as depression, and physical disorders, such as diabetes or cardiovascular problems, are associated with a higher risk of dementia. Inactivity, loneliness, and isolation are also associated with a higher risk of dementia.
Changes in sleep patterns may be an early sign of dementia itself. Some of the first areas of the brain affected by Alzheimer's are those that help regulate sleep and circadian rhythms. As a result, people who develop Alzheimer's disease may experience sleep problems even before showing signs of memory loss or other symptoms. Along with amyloid protein, the other main protein believed to cause Alzheimer's disease is called tau. Like amyloid, tau also accumulates in the brain and ultimately damages brain cells.
"Is lack of sleep enough to cause dementia? Perhaps not independently. But it certainly seems to be a risk factor for increasing the risk of dementia and perhaps also the rate of decline," explained Sudha Seshadri, founding director of the Glenn Biggs Institute for Alzheimer's and Neurodegenerative Diseases at the University of Texas Health Science Center at San Antonio. More research is required to fully understand these factors.
In another study, older adults who feel sleepy during the day and struggle with enthusiasm for activities may be more likely to develop motoric cognitive risk (MCR) syndrome, a syndrome related to dementia. The study included over 18,500 adult volunteers aged 50 and older, including 445 people over the age of 65 living in residential communities, all of whom had no record of dementia at the time of the study. At the start of the study, participants filled out detailed questionnaires about their sleep habits, focusing on six main sleep characteristics: short sleep, bad sleep quality, difficulty falling asleep, difficulty staying asleep, waking up early, and daytime sleepiness. They were also asked how often they had trouble staying awake while driving, eating meals, or engaging in social activities, and about their level of enthusiasm.
At the start of the study, 42 people had motoric cognitive risk syndrome. Over the course of the next three years, another 36 participants developed MCR. Among the participants, 177 were defined as "poor sleepers," or 39.8 percent of the cohort, and had a higher risk of developing MCR compared with "good sleepers." Of those with excessive daytime sleepiness and lack of enthusiasm, 35.5% developed motoric cognitive risk syndrome, compared to 6.7% of the people without those issues.
"There's potential that people could get help with their sleep issues and prevent cognitive decline later in life," said Dr. Victoire Leroy, MD, PhD, of Albert Einstein College of Medicine in the Bronx, New York. "Our findings emphasize the need for screening for sleep issues. More research needs to be done to look at the relationship between sleep issues and cognitive decline and the role played by motoric cognitive risk syndrome. We also need studies to explain the mechanisms that link these sleep disturbances to motoric cognitive risk syndrome and cognitive decline."
Once researchers adjusted for factors such as age, depression, and other health conditions, they found that people with excessive daytime sleepiness and lack of enthusiasm were more than three times more likely to develop MCR than those without these sleep-related problems. MCR syndrome was first described in 2013 and is now known to lead to dementia in some people. It often appears before dementia symptoms emerge, and older adults with MCR are believed to have twice the likelihood of developing dementia. The authors of the study noted that motoric cognitive risk syndrome doesn’t mean a person will automatically develop dementia, but there is an association between the two.
While the studies highlight significant associations between sleep issues and increased risk of developing dementia, they do not establish a cause-and-effect relationship. It's possible that dementia-related brain processes cause sleep loss, or that poor sleep contributes to the development of dementia. "Although there are clear associations, the authors note that this type of study doesn't prove that slow-wave sleep loss causes dementia." More research is needed to fully understand the mechanisms involved.
"It's important to prioritize sleep, aiming to sleep between seven and nine hours per night, starting at 40 or 50 years old, if not earlier," stated Joe Winer. "We don't have a great answer to whether sleep at 20 years old influences the risk one runs later in life, for example. But I think the indications point to perhaps in middle age, as one approaches 60 and 70 years old, sleep becomes important."
Sources: Newsweek, Home, McKnight's Long-Term Care News, Science Alert, Prensa Libre
This article was written in collaboration with generative AI company Alchemiq