Dementia is one of the leading causes of disability and mortality worldwide, and modifiable lifestyle factors may be critical for preventing up to 45% of all-cause dementia occurring after age 65. Two key modifiable risk factors associated with reduced dementia risk and improved brain health are regular physical activity and good quality sleep. Physical activity and sleep are unequivocally related: optimal sleep can positively impact energy levels for physical activity engagement, and physical activity can enhance sleep quality. However, there is limited research examining how physical activity and sleep may work together to influence brain health.
Our recent review proposed that greater engagement in physical activity has the potential to compensate for negative brain health consequences associated with poor sleep in older adults. This hypothesis was initially based on several cross-sectional studies from our research group and others demonstrating that greater physical activity is most closely associated with better cognition in those with poor sleep. Two randomized controlled trials, one in healthy older adults and one in patients with chronic stroke, have now also demonstrated that exercise is most effective for improving cognition in participants who reported poor sleep at study commencement. This suggests that among poor sleepers, remaining physically active may be particularly important for maintaining cognitive function and may protect against sleep-related cognitive deficits. It is notable that one recent longitudinal study demonstrated evidence to support this hypothesis cross-sectionally, though this was not supported when examining the data longitudinally. Further randomized controlled trials and longitudinal studies are required to clarify the strength of associations and long-term effects of exercise on cognition in individuals with poor sleep. Nevertheless, with approximately 50% of older adults reporting sleep difficulties and limited effectiveness of non-pharmaceutical sleep improvement interventions, the possibility that physical activity may reduce poor sleep burden on the brain could have significant public health implications. Our research further highlights to geriatricians and clinicians the importance of providing exercise prescriptions and encouraging increases in activity levels within older adult populations.
The optimal exercise prescription (e.g., frequency, intensity, duration) for potentially compensating sleep-related poorer cognition is yet to be determined. However, we have demonstrated moderate intensity aerobic exercise showed beneficial effects on cognition in a cohort of older adult (>60 years) poor sleepers, whereas high intensity aerobic exercise did not. This suggests that the compensatory influence of exercise may be intensity-dependent, and that prescription of exercise intensity should be carefully considered, particularly for poor sleepers.
The effect sizes for the influence of exercise on cognition is likely contingent on several methodological factors. For example, research suggests that moderators such as sleep quality may explain some of the heterogenous findings and weaken effect sizes. Many similar moderators have not been considered previous studies, which likely contributes to the muddiness and misconceptions within this scientific literature. Thus, future randomized controlled trials examining the effects of exercise on cognitive and brain health outcomes should consider using sleep quality as a moderator variable.
One interesting finding within our research is that the cognitive domain of episodic memory (memory of personal experiences) appears to be consistently influenced by sleep and physical activity. We also have demonstrated that physical activity and sleep might interact to influence the pathology underlying Alzheimer’s disease (brain beta-amyloid). Therefore, we hypothesize that sleep and physical activity might relate to both pathological and non-pathological brain changes in aging. However, the mechanisms underlying these associations require further investigation.
We hope that a better understanding of the synergistic association between sleep and physical activity will help optimize and individualize exercise prescriptions for preserving brain health in older adults.
Kelsey R. Sewell, PhD, is a postdoctoral research scientist in neuroscience at AdventHealth Research Institute, Orlando, Florida. Dr. Sewell earned her PhD at Murdoch University, Western Australia, where her training focused on sleep and physical activity patterns and how they influence brain health in older adults. Her broader work is focused on better understanding how lifestyle behavior optimization can prevent or delay the onset of Alzheimer’s disease and whether lifestyle changes can influence novel Alzheimer’s disease biomarkers (e.g., in blood). Dr. Sewell takes a comprehensive approach to her research, utilizing both large observational cohort studies and randomized clinical trials of exercise to investigate such questions.
