Sleeping Less Than 6 Hours Cuts Cognitive Benefits Of Exercise: Study

Sleeping less than six hours daily may cut down the protective effect of regular physical activity against cognitive decline, according to a study published in The Lancet Healthy Longevity journal. Researchers from University College London (UCL) in the UK looked at cognitive function over 10 years in 8,958 people aged 50 and over in England. They analysed how different combinations of sleep and physical activity habits might affect people's cognitive function over time.

The team found that people who were more physically active but had short sleeps – less than six hours on average – had faster cognitive decline overall, meaning that after 10 years their cognitive function was equivalent to peers who did less physical activity.

"Our study suggests that getting sufficient sleep may be required for us to get the full cognitive benefits of physical activity," said study lead author Mikaela Bloomberg from UCL Institute of Epidemiology & Health Care.

"It shows how important it is to consider sleep and physical activity together when thinking about cognitive health," Bloomberg said. 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.

The latest study found, in line with previous research, that sleeping between six and eight hours per night and higher levels of physical activity were linked to better cognitive function. Those who were more physically active also had better cognitive function regardless of how long they slept at the start of the study, the researchers said.

This changed over the 10-year period, with more physically active short sleepers (less than six hours) experiencing more rapid cognitive decline, they said. According to the researchers, this rapid decline was true for those in their 50s and 60s in this group, but for older participants (aged 70 and over) the cognitive benefits of exercise appeared to be maintained, despite short 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 Professor Andrew Steptoe from UCL Institute of Epidemiology & Health Care.

“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," Steptoe said.

The study participants were asked how long they slept on an average weeknight and were split into three sleep groups: short (less than six hours), optimal (six to eight hours) and long (greater than eight hours). The researchers noted some limitations in the study, as they relied on participants self-reporting their sleep duration and physical activity.

The next steps may be to repeat the results in more diverse study populations, examine more cognitive domains and more domains of sleep quality, and use objective measures such as a wearable physical activity tracker, the researchers added.