MONTREAL -- Middle-aged people who don't get enough sleep may have a higher risk of dementia 25 to 30 years down the line, a recent British study warns.

However, two Quebec experts consulted by The Canadian Press insist that doesn't mean that people in their 50s who get only five or six hours of sleep a night are automatically doomed to develop dementia by age 80, nor will those who get excellent sleep be automatically protected.

Sleep disorders and cognitive problems are multifactorial, they explain, and it can be very difficult to tell which is responsible for what.

The authors of the study, published in Nature Communications, followed some 8,000 people in the UK for 25 years, starting at the age of 50.

Subjects who consistently reported sleeping six hours or less per night were about 30 per cent more likely to be diagnosed with dementia some 30 years later, compared with subjects who slept seven hours per night.

The results take into account other factors that may influence the risk of dementia, such as smoking, diet, education, weight, physical inactivity and various health problems.

"The interesting thing about this study is that they started very early, at 50 years of age, probably before the neurodegenerative disease began to have its effects in the brain, which gives us the impression that we are beginning to touch on what could cause it," said Professor Sylvie Belleville, with the Institut universitaire de gériatrie de Montréal (IUGM). "This is a very interesting and important aspect of the study."

Neurodegenerative diseases, such as Parkinson's or Alzheimer's, begin to set in 15 or 20 years before the first physical symptoms appear, experts explain.

When faced with a 75-year-old Alzheimer's patient who sleeps poorly, for example, it can be very difficult to determine whether poor sleep is partly responsible for the onset of the disease, or whether it is the illness that causes the sleep problems.

This new study does not answer every question, but it is a very useful contribution to the debate, adds Dr. Thanh Dang-Vu, founder and director of the Sleep, Cognition and Neuroimaging Research Laboratory.

"This is another brick in the building that supports the fact that sleep is important and may be a significant risk factor for dementia," he said.


Low levels of education, as well as health problems like hypertension or diabetes have all been associated with an increased risk of dementia.

One study published in The Lancet recently suggested that eliminating certain risk factors could reduce the number of dementia cases worldwide by 40 per cent.

However, this new British study highlights a very interesting fact, notes Sylvie Belleville: all risk factors, including lack of sleep, correspond to a comparable risk ratio.

"People often ask us what they should work on to reduce their risk of dementia," she said. "It's not clear if there is one component that stands out. It obviously depends on your personal risk. If you sleep well, you don't need to work on that. If you're sedentary, you might want to work on that, but what we see is that there is not one thing that stands out, probably because the disease is very complex."

She adds there may be other, unidentified causes that could cause dementia, so the debate remains open.

It has been proven that during sleep, some of the proteins that eventually form the plaque associated with Alzheimer's disease are cleared out from the brain. In theory, that means insufficient sleep could prevent this type of "housekeeping" from being done properly.

"But we have to be realistic and accept that sleep is not the only factor linked to dementia," said Vu. "Although, it is probably partly responsible for the problem."

Could improving the quality of sleep reduce the risk of dementia?

"The question remains unanswered," he notes. "It's a modifiable risk factor that could be useful to prevent disease, but that's yet to be proven."

Vu draws a comparison with smoking: not all people who smoke will suffer from cardiovascular disease, and not all non-smokers will be spared these types of health problems.

"It's hard sometimes to tell the difference," admits Vu.

Both experts do agree on the importance of maintaining a healthy lifestyle for physical and mental health: eating well, moving and sleeping well.

"That's the message we need to get across. Sleep is part of the lifestyle habits that must be considered to promote optimal health," Vu concluded.

- This report by The Canadian Press was first published May 14, 2021.