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Lack of sleep may reduce the cognitive benefits of physical activity, study finds. Luke Mattson/Stocksy
  • Researchers looked at cognitive function over 10 years in 8,958 people aged 50 and older in England.
  • The study found people who slept between 6 and 8 hours per night and engaged in higher levels of physical activity were linked with better cognitive function.
  • People who slept fewer than 6 hours a night, even if they engaged in higher levels of physical activity, experienced more rapid cognitive decline over ten years.
  • Among participants aged 70 and older, the benefits of higher levels of physical activity on cognitive function appeared to be maintained despite the number of hours slept.

Evidence from existing research suggests that physical activities are beneficial to brain health and may protect against the development of neurodegenerative conditions such as dementia and Parkinson’s disease. However, a new study found that sleep deprivation can reduce such benefits reaped from exercise.

Almost 10% of adults age 65 and older in the United States have dementia, and another 22% have mild cognitive impairment, according to a 2022 nationally representative study of cognitive impairment prevalence.

Numerous studies have found exercise may reduce the risk of developing dementia. Still, more studies link a lack of sleep with increased dementia risk.

“Physical activity and sleep are factors that are thought to independently contribute to cognitive function, but they are also interrelated, where more physical activity is correlated with better quality sleep and physical activity may also regulate circadian rhythms,” Mikaela Bloomberg, Ph.D., a research fellow at the University College London Institute of Epidemiology and Health Care, explained to Medical News Today.

A team of UCL researchers, including Bloomberg, found little existing research that looked at the impact of physical activity and sleep on cognitive function. The studies they found were small and cross-sectional, which is a type of research where researchers collect data from participants at a single point in time.

“Because sleep disturbances can be an early symptom of neurocognitive diseases like dementia, which cause cognitive dysfunction, it is challenging to determine whether the results we observe in those previous studies are due to the effects of sleep on cognitive function or vice versa,” Dr. Bloomberg said. “With this in mind, we wanted to examine how combinations of physical activity and sleep habits influenced cognitive function over a long period of time.”

A paper by the UCL researchers on their large-scale, longitudinal study appears in The Lancet Healthy Longevity

For their study, UCL researchers used longitudinal data on 8,958 cognitively healthy adults from England aged 50 and older taken from the English Longitudinal Study of Ageing (ELSA). The data used was collected between Jan. 1, 2008, and July 31, 2019.

Participants gave reports about their physical activity and sleep duration every two years.

Researchers asked participants how many hours they slept on a typical weeknight. The UCL researchers then categorized sleep as “short” if it was less than six hours, “optimal” if between six and eight hours, and “long” if more than eight hours were received.

Researchers also asked participants how much they exercised. Participants reported how frequently they participated in light, moderate, and vigorous physical activity and whether they exercised more than weekly, weekly, one to three times a month, and rarely/never.

Researchers assessed the episodic memory of participants using the Consortium to Establish a Registry for Alzheimer’s disease immediate and delayed recall tasks. Researchers gave participants a ten-word list and asked them to recall the words immediately and again a day later. Researchers also assessed the participants’ verbal fluency using a task where the participants named as many animals as they could think of over the course of a minute.

The UCL researchers excluded participants who reported being diagnosed with dementia during the follow-up period as well as participants whose test scores suggested some cognitive impairment. Additionally, researchers adjusted their analyses for a number of factors, such as whether participants had taken the same cognitive test previously.

Of the 3,069 participants who researchers placed in the “higher physical activity category,” 1,525 participants (50%) reported engaging in light, moderate, and vigorous exercise more than weekly. Another 1,161 participants (37.8%) reported engaging in light and moderate exercise more than weekly and vigorous exercise monthly or weekly.

Among the 5,889 participants in the lower physical activity category, 2,384 participants (40.5%) reported engaging in no vigorous physical activity but more than weekly light and moderate physical activity. Another 1,511 participants (25.7%) reported engaging in more than weekly light physical activity, moderate physical activity weekly or less often, and no vigorous physical activity.

Participants who engaged in higher physical activity were more likely to sleep 6–8 hours a night. They were also more likely to be younger at baseline, male, married, or had a partner, and had more education and wealth than those in the lower physical activity group. Those in the higher physical activity group were more likely not to smoke, had lower body mass indexes (BMI), fewer diagnoses of all chronic conditions, and fewer depressive symptoms compared with those in the lower physical activity group.

Participants from the higher physical activity group generally had the highest baseline cognitive scores regardless of how long they slept.

“[H]owever, for ages 50 and 60 years, those with higher physical activity and short sleep declined more rapidly such that after 10 years of follow-up, they had cognitive scores similar to those in the lower physical activity groups,” the UCL researchers write in their paper about the study.

“We were surprised to see that the cognitive benefits associated with physical activity were reduced when participants had insufficient sleep duration, but these findings are certainly in line with previous research pointing to an important role of sleep in cognitive and physical recovery.”

– Dr. Bloomberg

Among older participants (age 70 and above) the cognitive benefits of exercise appeared to be maintained even among poor sleepers.

Dr. Vernon Williams, sports neurologist, pain management specialist, and founding director of the Center for Sports Neurology and Pain Medicine at Cedars-Sinai Kerlan-Jobe Institute in Los Angeles, told MNT he appreciated seeing data about the importance of sleep for long-term benefits regarding cognitive function.

“The concept that both exercise AND sleep are critical factors for maintaining cognitive health coupled with evidence that maintaining physical health in the absence of optimal sleep health reduces the cognitive benefits of physical activity is compelling,” Dr. Williams said.

Ryan Glatt, a senior brain health coach and director of the FitBrain Program at the Pacific Neuroscience Institute in Santa Monica, CA, told MNT he found the study “very interesting” but noted limitations.

“[T]here are potential issues with the accuracy of self-reported physical activity and sleep duration, and the potential presence of sleep disorders or the effects of certain medications were not considered,” Glatt said.

Dr. Bloomberg believes there may be a way to conduct this research that doesn’t rely on participants’ truthfulness.

“An interesting next step would be to use objective measures of sleep and physical activity — for example, using wrist worn accelerometers —to see whether we observe similar results,” she told MNT.

In the future, the UCL researchers would also like to see a similar study performed on more diverse populations. Additionally, Dr. Bloomberg told MNT she’d like “to extend the results to dementia.”

“We purposefully excluded those with dementia and those with cognitive scores that suggested cognitive impairment, in order to make it more likely that we were capturing the effects of sleep on cognitive function and not vice versa,” Dr. Bloomberg said. “Future research should [examine] how combinations of physical activity and sleep impact [the] risk of dementia.”