April 22, 2021 -- People who sleep for less than 6 hours a night in their 50s and 60s appear more likely to develop dementia than their peers who get at least 7 hours of shut eye a night, according to a new study.

"We showed that a consistent association between short sleep duration in midlife and risk of dementia. This association was not explained by mental disorders and other chronic conditions known to be associated with dementia," Séverine Sabia, PhD, from the University of Paris, , said. .

"This study highlights the importance of having good sleep hygiene for brain health," Sabia added.

The study was published online April 20 in Nature Communications.

Novel Study

"In studies on dementia, it is important to keep in mind the long preclinical period of dementia," Sabia noted.

As most dementias are characterized by various functional changes over 20 years or more, studies with a long follow-up are needed to gain insight into the association between sleep duration and subsequent dementia.

Until now, much of the evidence on this connection came from studies with follow-up lasting less than 10 years.

For this study, however, investigators looked at the connection between sleep and dementia in 7,959 participants from the U.K. who were followed for 25 years.

During follow-up, there were 521 diagnosed cases of dementia.

The results showed a higher dementia risk associated with a sleep habit of 6 hours or less at age 50 and 60, compared with those who got 7 hours or more per night.

There was no clear evidence of an association between long sleep duration and incident dementia, as has been reported in some studies.

Strong Supportive Evidence

"There is intense interest in whether poor sleep could cause or worsen dementia and, therefore, whether improving sleep might help prevent dementia," Elizabeth Coulthard, PhD, senior lecturer in dementia neurology at the University of Bristol, said in a statement from the U.K. nonprofit Science Media Centre.

"Before this study there was already strong evidence that sleep becomes abnormal before dementia is diagnosed. But this still does not tell us whether sleep triggers or exacerbates dementia because the brain changes that cause dementia start many years before a diagnosis," said Coulthard, who was not involved in the study.

This new study, said Coulthard, "adds new information to the emerging picture" because sleep is reported in a middle-aged cohort followed for more than two decades.

"This means that at least some of the people who went on to develop dementia probably did not already have it at the start of the study when their sleep was first assessed. So it strengthens the evidence that poor sleep in middle age could cause or worsen dementia in later life," Coulthard said.

Also weighing in on the study, Tom Dening, MD, head of the Centre for Dementia at the Institute of Mental Health at the University of Nottingham in the U.K, said there is evidence that sleep disturbance can occur a long time before the onset of other evidence of dementia.

"However, this study cannot establish cause and effect," Dening cautioned.

"Maybe it is simply a very early sign of the dementia that is to come, but it's also quite likely that poor sleep is not good for the brain and leaves it vulnerable to neurodegenerative conditions like Alzheimer's disease," Dening said.

"Turning off mobile phones and avoiding caffeine before bed are good habits to have as we already know the importance of good sleep on health more generally. However, we would need further studies to know if longer sleep in itself could reduce the risk of dementia later in life," he added.