Abstract Engaging in regular physical activity and obtaining recommended amounts of sleep are touted as strategies to promote healthy brain aging. However, as each day is only 24 hours long, changing time spent in one activity must come at the expense or gain of another, making it necessary to understand how the whole 24-hour activity composition impacts dementia risk. We applied compositional data analysis to investigate the effect of substituting sleep duration for different levels of physical activity (i.e., inactivity, light activity, and moderate to vigorous physical activity; MVPA) on dementia risk relative to two reference compositions; a “typical” short sleeper (< 6hrs) and normal sleeper (≥ 6hrs). The study sample comprised participants from the community-based UK Biobank with 24-hour behaviors estimated using 7 days of accelerometry. The mean age of the sample was 63 years (Q1, Q3: 56, 68); 56% were women. Of the 88,654 participants, there were 718 incident all-cause dementia cases over a median follow-up of 8.2 years. For short sleepers, increasing sleep duration at the expense of inactivity or light activity was associated with a lowering of dementia risk, but not when at the expense of MVPA. For persons with normal sleep duration, the effect of increasing or decreasing sleep duration on dementia risk differed for all three substituted behaviors (i.e., inactivity, light, or MVPA). Most notably, dementia risk was higher when increasing sleep at the expense of MVPA and lower when increasing MVPA at the expense of sleep. The interpretation of the results was broadly consistent when using MRI-based outcomes (e.g., hippocampal volume) in a subset with brain imaging (n = 15,263). Our findings underscore the complexity of optimizing dementia risk reduction strategies, emphasizing the need for personalized approaches that balance trade-offs between sleep duration and differing physical activity levels based on individual circumstances, such as habitual sleep duration.
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