Exercise may stave off dementia, but only in combination with sufficient sleep, research has found. A team from UCL followed 9,000 people in England aged 50 and above for a decade. Those who got between six and eight hours sleep fared better on tests of cognitive performance, as did the most physically active participants.
However, people in their fifties and sixties who did a lot of exercise, but got less than six hours of sleep on average, saw rapid cognitive decline over the ten years, akin to those who were less active.
Professor Andrew Steptoe of UCL’s Institute of Epidemiology and Health Care, a co-author on the paper, said: “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.
“The World Health Organisation already identifies physical activity as a way to maintain cognitive function, but interventions should also consider sleep habits to maximise long-term benefits for cognitive health.”
For those aged over 70, the benefits of exercise continued even without sleep.
The study is published in The Lancet Healthy Longevity. It used data from the English longitudinal study of ageing, which recorded how long people slept and how physically active they were.
Those who slept less than six hours on an average weeknight were classed as “short” sleepers, those who got six to eight hours “optimal”, and those who got more than eight hours “long”.
The top third most active, in terms of frequency and intensity, were compared with the less active other two thirds.
Cognitive function was assessed at the beginning and end of the study with a test that asked participants to recall a ten-word list, both immediately and after delay, and another that asked them to name as many animals as they could in a minute.
When the study began, those who had higher physical activity and optimal sleep scored higher on cognitive tests than any of the other groups. Within the most active group, there was no difference in cognitive performance between sleep categories.
But after ten years, those with higher levels of physical activity but short sleep habits had faster rates of cognitive decline than their counterparts who slept for longer.
The researchers reported that this was, “such that their scores at ten years were commensurate with those who reported low physical activity, regardless of sleep duration”.
The lead author, Dr Mikaela Bloomberg, also of UCL, said: “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.”
She said previous studies looking at how sleep and physical activity worked together to affect cognitive function had tended to only focus on a snapshot in time. “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,” she said.