The protective effect of regular physical activity against age-related cognitive decline was diminished among older adults who did not get enough sleep, according to a new study of nearly 9000 older adults.
Researchers from University College London (UCL) said that physical activity interventions "should also consider sleep habits, to maximise benefits of physical activity for long-term cognitive health".
Alzheimer's disease and related dementias were leading causes of cognitive dysfunction and disability in older adults, the researchers noted. Among the multiple potentially contributory lifestyle factors to poor cognitive function were lower physical activity levels and poor sleep, both of which are associated with worse cognitive performance.
For their study, published in Lancet Healthy Longevity, they used data from the English Longitudinal Study of Ageing, a nationally representative cohort study now in its 20th year, to explore how physical activity and sleep interacted to influence cognitive ageing. Data were collected collected between 1 January 2008 and 31 July 2019 for 8958 respondents aged 50-95 years at baseline, with follow-up interviews every 2 years for a median follow-up of 10 years.
At each interview, episodic memory was assessed using immediate and delayed recall tasks, and verbal fluency using an animal naming task; scores were standardised and averaged to produce a composite cognitive score. Participants were also given a score based on self-reported physical activity and divided into two groups: more physically active (the top third of scorers) and less physically active (the other two thirds), based on both frequency and intensity of activity. In addition, they were asked about sleep duration on an average weeknight, which was categorised as short (<6 hours), optimal (6-8 hours), or long (>8 hours).
The researchers adjusted for a number of confounding factors, such as participants having done the same cognitive test before and thus being likely to perform better. After excluding people with a self-reported dementia diagnosis and those whose test scores indicated some cognitive impairment, so that behaviour changes linked to preclinical Alzheimer's disease (such as sleep disturbance) did not inadvertently affect the results, the researchers then associated the rate of cognitive decline after 10 years with independent and joint measures of physical activity and sleep duration.
Sleep and Physical Activity Independently Linked to Cognitive Function
They found, in line with previous research, a U-shaped association between sleep duration and cognitive performance. Both sleeping between 6 and 8 hours per night and higher levels of physical activity were independently linked to better cognitive function.
At baseline, participants with higher physical activity and optimal sleep had higher cognitive scores than all combinations of lower physical activity and sleep categories. Those participants who were more physically active at the start of the study had better cognitive function, regardless of how long they slept. However over the 10-year period, the cognitive trajectories changed, with those sleeping for less than 6 hours experiencing more rapid cognitive decline, even among the more physically active group.
After 10 years, the cognitive function of physically active short sleepers – those who had less than 6 hours per night on average – had declined faster than that of longer sleepers, and was equivalent to that of their peers who did less physical activity, the researchers found.
The rapid decline held for those in their 50s and 60s, although for participants aged 70 and over in this group, the cognitive benefits of exercise appeared to be maintained despite short sleep.
Furthermore, sex stratified results for men and women separately were similar to those of the main analysis, with the exception of long sleep, which was associated with a more favourable cognitive trajectory among men only, as were the cognitive benefits associated with higher physical activity combined with long sleep. This indicated that "determinants of long sleep might differ between men and women", the team said, suggesting that this warranted further research.
Cognitive Decline Associated with Short Sleep Despite Physical Activity
They concluded: "The baseline cognitive benefit associated with more frequent, higher intensity physical activity was insufficient to ameliorate the more rapid cognitive decline associated with short sleep."
Lead author Dr Mikaela Bloomberg PhD, a research fellow at UCL's Institute of Epidemiology & Health Care, said: "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 explained: "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."
Co-author Andrew Steptoe, professor of psychology and epidemiology at the Institute, 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."
He noted that while physical activity was recognised as a way to maintain cognitive function, interventions to promote it "should also consider sleep habits to maximise long-term benefits for cognitive health".
Asked to comment by Medscape News UK, Caroline Abrahams, charity director for Age UK, said: "These results are interesting as they highlight how complex it can be to manage lifestyle factors that impact upon cognitive decline in later life. We know that it's important both to be physically active as we age, as well as to get sufficient sleep.
"In maintaining a healthy lifestyle we should make sure that we get enough of both in our 50s and 60s. It's often harder to feel like we're getting enough sleep as we move into later life, so it's encouraging to see that older people aged 70 and over are able to maintain the cognitive benefits from physical activity, even if they are getting less than 6 hours sleep."
The research was funded by the UK’s Economic and Social Research Council. The authors declared no conflicts of interests.