The link between too little sleep and a higher risk of dementia is well documented. Now, a new study out of Japan finds that the opposite may also be true: Too much sleep can raise your risk of cognitive decline, dementia, and even early death.

A team of researchers from Kyushu University followed the sleep habits of 1,517 adults age 60 and over for 10 years, none of whom had signs of cognitive impairment before the study began.

They found that participants who slept 10 or more hours a day were more than twice as likely to develop dementia or die, compared to those who slept between five and 6.9 hours—roughly the same risk level associated with sleeping less than five hours a day. The results were the same regardless of the age or sex of the participant.

The researchers did not determine what caused the heightened risk.

Exploring the possible reasons

The Japanese team did, however, point to two possibilities that merit further research.

Poor sleep quality, the study noted, may cause a protein called amyloid that has been associated with developing Alzheimer’s disease to build up in the brain. Sleep is one way this protein is cleared from the body.

People who slept for 10 or more hours were twice as likely to develop dementia or die early.
Kyushu University

The researchers also noted that sleep disturbances could cause low-grade inflammation in the brain, which has been linked to diabetes, cancer, depression, and other ailments. These, in turn, are all known risk factors for dementia or death.

The risks grow as you get older.

As people age, they are more likely to experience changes in their sleep patterns, says geriatric sleep researcher Constance Fung, an associate professor of medicine at the David Geffen School of Medicine at UCLA. Aging can also cause changes to the circadian system, or the biological clock, Fung says, which can contribute to impaired memory.

Other studies have also found a connection between developing dementia and sleeping too much or too little.

One published in the journal Neurology, for example, concluded that sleeping for more than nine hours a day may be an early sign of dementia or Alzheimer’s.

One possible explanation, according to Fung, may be that inflammation, which can occur with the dementia process, leads to increased sleepiness and sleeping longer.

How you can lower your risk

Although the Japanese study didn’t establish why too much shuteye is linked to a greater risk of dementia and early death, the research did suggest a couple of lifestyle changes that may be helpful in reducing the danger.

First, get a moderate amount of exercise.

The study found that a low level of activity was linked to a higher risk of cognitive decline and early death among both those who slept too much and those who got too little rest.

Taking sleeping pills was linked to a 66% greater chance of developing dementia.
Kyushu University

High levels of physical activity also lowered the risks of both dementia and early death among people who got less than five hours of sleep a night. Among those who slept 10 hours or more, however, high physical activity didn’t lower the risk of dementia but did seem to erase the heightened risk of premature death.

This suggests taking a Goldilocks approach—not too much physical activity or too little, but just the right amount. For moderate exercise, the Centers for Disease Control and Prevention recommend 150 minutes a week (say, half an hour, five days week) of activities such as walking briskly, water aerobics, bicycling, ballroom dancing, or gardening.

Just say no to drugs (mostly)

The Japanese study findings also suggest medication to help you sleep should be used only as a last resort—if you use them at all.

Participants who took sleeping pills were 66% more likely to develop dementia and 83% more likely to die early than those who did not take drugs to help them rest. That was true regardless of how much sleep they got each night.

To get a healthy five to eight hours of shuteye each night without pharmaceutical aids, try to maintain a sleep routine, like going to bed and waking up at around the same time each day. The American Sleep Association also suggests avoiding naps and exercising regularly before 2 p.m.

And, adds Fung, if you still have problems staying asleep, sleeping too long, or are gasping during sleep, work with your healthcare provider to find a treatment that is effective for you.