The Goldilocks zone: How poor sleep affects your risk of dementia

While scientists are confident that a connection between sleep and dementia exists, the nature of that connection is complicated. Illustration / Holly Stapleton, The New York Times

Getting too little sleep later in life is associated with an increased risk for Alzheimer’s disease. But paradoxically, so is getting too much sleep.

While scientists are confident that a connection between sleep and dementia exists, the nature of that connection is complicated. It could be that poor sleep triggers changes in the brain that cause dementia. Or people’s sleep might be disrupted because of an underlying health issue that also affects brain health. And changes in sleep patterns can be an early sign of dementia itself.

Here’s how experts think about these various connections and how to gauge your risk based on your own sleep habits.

Too little sleep

Sleep acts like a nightly shower for the brain, washing away the cellular waste that accumulates during the day. During this process, the fluid that surrounds brain cells flushes out molecular garbage and transfers it into the bloodstream, where it’s then filtered by the liver and kidneys and expelled from the body.

That trash includes the protein amyloid, which is 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 accumulates into sticky clumps, called plaques. The longer someone is awake, the more amyloid builds up and the less time the brain has to remove it.

Scientists don’t know whether regularly getting too little sleep — typically considered six hours or less a night — is enough to trigger the accumulation of amyloid on its own. But research has found that among 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 sufficient to cause dementia? Probably not by itself alone,” said Dr Sudha Seshadri, the founding director of the Glenn Biggs Institute for Alzheimer’s and Neurodegenerative Diseases at the University of Texas Health Science Center at San Antonio. “But it seems to definitely be a risk factor for increasing the risk of dementia, and perhaps also the speed of decline.”

People with Alzheimer’s disease may start to develop symptoms in their 60s or 70s, but amyloid can begin to accumulate up to two decades earlier. That’s why it’s important to prioritise sleep, aiming for seven to nine hours a night, starting in your 40s or 50s, if not before, said Joe Winer, a postdoctoral fellow of neurology and neurological sciences in the Center for Sleep and Circadian Sciences at Stanford University.

Everyone’s brain produces a protein called amyloid during the day, but problems can arise when it accumulates into sticky clumps, called plaques. Photo / 123RF

“We don’t have a great answer to, like, does your sleep in your 20s impact your late-life risk?” Winer said. “But I think that the signs point to probably in midlife, as you head toward your 60s and 70s, your sleep is going to be important.”

Some sleep disorders, most notably sleep apnea, are also associated with an increased risk for dementia. That may be because sleep apnea disrupts people’s sleep, or because it tends to occur in people who are overweight or have diabetes, which are also linked to dementia.

But even when you remove the effect of these other issues, sleep apnea appears to confer its own, independent risk for dementia, said Dr Diego Carvalho, an assistant professor of neurology at the Mayo Clinic’s 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

On the other end of the spectrum, too much sleep also appears to be linked to an increased risk for dementia, though perhaps more indirectly.

If a person is regularly staying in bed for more than nine hours a night, or taking multiple naps during the day, it may be a sign that they are sleeping very poorly, which could raise the risk for Alzheimer’s disease for the reasons listed above.

Alternatively, needing excess sleep might be related to a mental or physical disability. Mental health conditions, like depression, and physical health conditions, such as diabetes or cardiovascular problems, are associated with a higher risk for dementia, as are physical inactivity, loneliness and isolation.

“At this point, there’s no clear causative role of long sleep in relationship to dementia,” Carvalho said. “It may be more like a symptom of an underlying problem than the cause of the problem.”

An early symptom?

Some of the first areas in the brain affected by Alzheimer’s are those that help regulate sleep and circadian rhythms. As a result, people who develop the condition may experience sleep problems even before they show signs of memory loss or other symptoms.

Along with amyloid, the other main protein thought to cause Alzheimer’s disease is called tau. Like amyloid, tau also accumulates in the brain, ultimately damaging brain cells. One of the first places that tau buildup appears “are these brainstem areas important for regulating sleep and wake”, Winer said. “So we think that tau showing up in these areas very early on is going to disrupt people’s sleep-wake cycles.”

Sleep problems can be an early sign of other common types of dementia, as well. In Lewy body dementia and Parkinson’s disease dementia, for example, rapid eye movement sleep is sometimes disrupted, causing people to act out their dreams — something your bed partner might notice, Seshadri said.

“Normally, when you have this rapid eye movement sleep, that is time when the body’s muscle tone goes down almost to zero, and so the muscles don’t move,” she explained. “In the REM behaviour disorder, this suppression of muscle tone is lost, and so the muscles actually move the way you are in your dreams.”

Experts say it’s normal for older adults to sleep a little more or a little less after retirement, or to wake up and go to bed a little earlier or later than they used to. But if there’s been a dramatic shift, consider seeing your doctor or a sleep specialist.

“If somebody is getting up at 2am or 3am, or is sleeping for three hours during the day, that’s a cause for concern,” Seshadri said. “If somebody is getting up an hour earlier than they used to and having maybe a 30- to 60-minute nap during the day, that’s much more likely to be just normal aging.”

This article originally appeared in The New York Times.

Written by: Dana G. Smith

Illustration by: Holly Stapleton

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