Sleeping too little in old age is associated with an increased risk of Alzheimer’s disease. But paradoxically, so is sleeping too much.
Although scientists are convinced of a link between sleep and dementia, the nature of that link is complex. It could be that poor sleep triggers changes in the brain that cause dementia. Or people’s sleep may be disrupted because of an underlying health condition that can also cause dementia. affects brain health. And changes in sleep patterns can be an early sign of dementia itself.
Here’s how experts view these different links and how to assess your risk based on your own sleep habits.
Too little sleep
Sleep acts as a nighttime shower for the brain, flushing out cellular waste accumulated during the day. During this process, the fluid surrounding brain cells removes molecular waste and transfers it into the bloodstream, where it is then filtered by the liver and kidneys and expelled from the body.
These waste products include amyloid, a protein thought to play a key role in Alzheimer’s disease. Everyone’s brain produces amyloid during the day, but problems can arise when the protein builds up in sticky clumps called plaques. The longer a person is awake, the more amyloid builds up and the less time the brain has to clear it.
Scientists aren’t sure whether lack of sleep (typically six hours or less a night) alone is enough to trigger amyloid buildup. But research has shown that in adults ages 65 to 85 who already have plaques in their brains, the less sleep they got, the more amyloid was present and the worse their cognition.
“Is lack of sleep enough to cause dementia? Probably not on its own,” said Dr. Sudha Seshadri, founding director of the Glenn Biggs Institute for Alzheimer’s and Neurodegenerative Diseases at the University of Texas Health Science Center in San Antonio. “But it does appear to be a risk factor for increasing the risk of dementia, and perhaps also the rate of decline.”
People with Alzheimer’s disease can start developing symptoms in their 60s or 70s, but amyloid can start accumulating up to 20 years earlier. That’s why it’s important to prioritize sleep, aiming for seven to nine hours a night, starting in your 40s or 50s, or even earlier, said Joe Winer, a postdoctoral fellow in neurology and neurological sciences at Stanford University’s Center for Sleep and Circadian Sciences.
“We don’t have a clear answer to the question of whether sleep in your 20s impacts risk later in life,” Dr. Winer said. “But I think the evidence probably indicates that in middle age, in your 60s or 70s, sleep is going to be important.”
Some sleep disorders, including sleep apnea, are also associated with an increased risk of dementia. This may be because sleep apnea disrupts people’s sleep, or because it tends to occur in people who are overweight or have diabetes, both of which are also linked to dementia.
But even when these other problems are taken into account, sleep apnea appears to carry an independent risk for dementia, said Dr. Diego Carvalho, an assistant professor of neurology at the Mayo Clinic Center for Sleep Medicine. That 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.
Too much sleep
At the other end of the spectrum, too much sleep also appears to be linked to an increased risk of dementia, although perhaps more indirectly.
If a person regularly stays in bed for more than nine hours a night or takes several naps during the day, this may be a sign that they are sleeping very poorly, which could increase the risk of Alzheimer’s disease for the reasons listed above.
Excessive sleep needs may also be linked to mental or physical disabilities. Mental health problems, such as depression, and physical health problems, such as diabetes or cardiovascular problems, are associated with a higher risk of dementia, as are physical inactivity, loneliness and isolation.
“At this point, there is no clear causal link between prolonged sleep and dementia,” Dr. Carvalho said. “It may be more of a symptom of an underlying problem than the cause of the problem.”
An early symptom?
The first areas of the brain affected by Alzheimer’s disease are those that help regulate sleep and circadian rhythms. As a result, people with the disease may experience sleep problems even before they show signs of memory loss or other symptoms.
Besides amyloid, the other major protein suspected of causing Alzheimer’s is called tau. Like amyloid, tau also accumulates in the brain, eventually damaging brain cells. One of the first places tau builds up “are these areas of the brainstem that are important for regulating sleep and wakefulness,” Dr. Winer said. “So we think that the very early appearance of tau in these areas is going to disrupt people’s sleep-wake cycles.”
Sleep disturbances can also be an early sign of other common types of dementia. In Lewy body dementia and Parkinson’s disease, for example, rapid eye movement sleep is sometimes disrupted, causing people to act out their dreams, something your bed partner might notice, Dr. Seshadri said.
“Normally, when you have this REM sleep, that’s when the muscle tone in the body drops to almost zero, and so the muscles don’t move,” she explains. “In REM behavior disorder, that suppression of muscle tone is lost, and so the muscles actually move like you do in your dreams.”
Experts say it’s normal for older adults to sleep a little more or a little less after retirement, or to rise and go to bed a little earlier or later than before. But if this drastic change occurs, consider consulting your doctor or a sleep specialist.
“If someone is waking up at 2 or 3 a.m. or sleeping three hours a day, that’s a cause for concern,” Dr. Seshadri said. “If someone is waking up an hour earlier than they used to and taking a 30- to 60-minute nap during the day, it’s much more likely that it’s just normal aging.”