Abstract Background 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 h 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-h activity composition is associated with dementia risk. We investigated the effect of substituting sleep duration for different levels of physical activity (i.e., inactivity, light activity, and moderate to vigorous physical activity; MVPA) in short sleepers (< 6 h) and normal sleepers (≥ 6 h and ≤ 9 h). Methods The study sample comprised 87,490 participants from the community-based UK Biobank, with 24-h behaviors estimated using up to 7 days of accelerometry. Participants were free from dementia or severe neurological disease at baseline. The main outcome was the risk of incident all-cause dementia over a median follow-up of 8.2 years. Results The mean age of the sample was 63 years (Q1, Q3, 56, 68); 56% were women. For short sleepers, increasing sleep duration was associated with a lowering of dementia risk when at the expense of inactivity or light activity, but not when at the expense of MVPA. For normal sleepers, 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, increasing sleep at the expense of MVPA was associated with greater dementia risk, and increasing MVPA at the expense of sleep was associated with lower dementia risk. The interpretation of the results was broadly consistent when using MRI-based outcomes (e.g., hippocampal volume) in a subset with brain imaging ( n = 15,180). Conclusions Our findings from this observational analysis suggest that personalized approaches that balance trade-offs between sleep duration and differing physical activity levels based on individual circumstances, such as habitual sleep duration, may be important for dementia risk reduction.
Yiallourou et al. (2025) studied this question.