Getting regular exercise can help protect against mental decline in an aging brain. But poor sleep can take away those benefits.

A new study found that people who were more active but slept less than six hours on average had faster cognitive (mental) decline. After 10 years, their cognitive function was equivalent to that of their more inactive peers.

“Our study suggests that getting sufficient sleep may be required for us to get the full cognitive benefits of physical activity. It shows how important it is to consider sleep and physical activity together when thinking about cognitive health,” said lead author Dr. Mikaela Bloomberg, of University College London (UCL) Institute of Epidemiology & Health Care, in England.

“Previous studies examining how sleep and physical activity might combine to affect cognitive function have primarily been cross-sectional — only focusing on a snapshot in time — and we were surprised that regular physical activity may not always be sufficient to counter the long-term effects of lack of sleep on cognitive health,” Bloomberg added in a university news release.

The new study included more than 8,950 people in England who were aged 50 and older.

Participants were asked how much sleep they get on an average weeknight and were divided into three groups: those who averaged fewer than six hours; those who got the optimal six to eight hours; and long-sleepers, those averaging more than eight hours each night.

Each was also scored based on the frequency and intensity of self-reported physical activity and divided into two groups: more active (top-third of scorers), and less active.

To assess participants’ mental function, they were asked to recall a 10-word list, both immediately and after a delay. They also took a verbal fluency test, in which they were asked to name as many animals as they could in a minute.

People were excluded if they self-reported a dementia diagnosis or had test scores indicating some mental impairment.

The study found that six to eight hours of sleep per night and higher levels of physical activity were linked to better cognitive function.

Those who were more active also had better mental function no matter how long they slept when the study began. The researchers noted that this changed over the 10-year study period, with those sleeping fewer than six hours having more rapid decline.

This decline was true for folks in their 50s and 60s, but those 70 and up maintained the mental benefits of exercise even with shorter sleep.

“It is important to identify the factors that can protect cognitive function in middle and later life as they can serve to prolong our cognitively healthy years and, for some people, delay a dementia diagnosis,” said study co-author Andrew Steptoe of the UCL Institute of Epidemiology & Health Care. “The World Health Organization already identifies physical activity as a way to maintain cognitive function, but interventions should also consider sleep habits to maximize long-term benefits for cognitive health.”

One limitation of the study is that participants self-reported on their sleep duration and physical activity.

The findings were published in the July issue of the The Lancet Healthy Longevity.

More information

The U.S. National Institute on Aging has more on mild cognitive impairment.

SOURCE: University College London, news release, July 5, 2023