Mikeala Bloomberg, Research Fellow at the UCL Institute of Epidemiology and Health Care, shares about her recent study which looked at the significance of physical activity and sleep for cognitive benefits.
In our recent study of adults aged 50 years and above, we found that though regular participation in high intensity physical activity was associated with cognitive benefits, if physical activity was not supported by sufficient sleep, these cognitive benefits would not necessarily last long term.
Each 24-hour day is broadly composed of time spent engaging in physical activity, being sedentary, and sleeping. Though the composition of our days differs substantially from person to person, there are several changes that tend to occur as we age: sedentary behaviour increases, physical activity decreases, and the quality and duration of our sleep changes.
As we get older, we might be less likely to engage in regular high intensity physical activity (for example, running or fast cycling) due to physical or time constraints, and simultaneously our sleep schedules change. Older adults may have more difficulty getting to sleep or staying asleep, may wake up frequently during the night, or may sleep for longer due to disorders like sleep apnoea or other conditions that make getting a good night’s sleep challenging.
While the main factor that regulates our sleep-wake cycles is light exposure, physical activity may also play a role. As we reduce the frequency and intensity of our physical activity, it can impact our sleep, which can then leave less energy to engage in physical activity, and so the cycle continues.
While this feedback loop of sleep and physical activity is ongoing, our cognitive abilities can also be impacted. It is well-established that poor sleep quality might reduce our cognitive performance the following day.
However, poor sleep quality also has long term implications for cognitive function, where short sleep duration (generally less than six hours per night) has been associated with a faster decline in cognitive abilities with age and increased risk of dementia. There is also evidence that regular exercise can enhance short-term cognitive performance and may be protective against cognitive decline.
Even though sleep and physical activity are inextricably linked, there are few studies that examine how they combine to influence cognitive decline over long periods of time. This is an important gap in the knowledge base because sleep and physical activity behaviours may change preceding dementia diagnosis. When we aim to examine the combined effects of sleep and physical activity on cognitive function, we may instead be observing the effects of poor cognitive function on sleep and physical activity.
In our study, we used data from the English Longitudinal Study of Ageing, a large population-based cohort study of adults aged 50 years and older. Cognitively healthy participants were asked about their sleep and physical activity habits in 2008, and were then given cognitive tests every two years until 2018 to see how cognitive function changed during the 10-year period for people with different combinations of sleep duration and physical activity habits.
Excluding those who reported dementia diagnosis during the follow-up period or who had cognitive scores that suggested cognitive impairment, as well as examining long term changes in cognitive function made it more likely that we were observing the effects of sleep and physical activity on cognitive function rather than the other way around.
Unsurprisingly, we found that sleeping 6-8 hours per night and more frequent higher intensity physical activity were associated with better cognitive performance, and indeed at the baseline of the study, people with more physical activity generally outperformed those with less physical activity regardless of sleep quality.
However, we also noted that adults who were in middle or early old age at baseline with more physical activity but short sleep duration (less than 6 hours) showed more rapid cognitive decline over the 10-year period than the other high physical activity groups, to the extent that by the end of the 10-year period their cognitive performance was similar to those in the low physical activity group.
While it is not clear precisely what is driving these results, it is possible that individuals who participate in a lot of physical activity just require more sleep, and when those sleep needs aren’t met, we observe deterioration in cognitive abilities over the long term. This result also points to the importance of considering sleep and physical activity habits together as regular participation in high intensity physical activity may not make up for poor sleep habits.
The results of this study suggest that it is important to support physical activity habits with sufficient sleep to maintain the long-term cognitive benefits of physical activity. Interventions to improve cognitive function that target physical activity habits may be less effective if they do not also consider sleep habits.
It remains to be seen whether this result will hold consistent when objective rather than subjectively assessed sleep and physical activity are examined, and if we will see a similar relationship between sleep, physical activity, and dementia.
The research presented in this blog post is based on Mikaela’s recent publication which can viewed online.
About the author
Mikaela Bloomberg is a Research Fellow at the UCL Institute of Epidemiology and Health Care. Her research focusses on sociodemographic and lifestyle determinants of cognitive ageing and dementia.