Associations between the 24-Hour Activity Daily Cycle and Incident Dementia.

BACKGROUND Physical activity, sedentary behavior, and sleep all impact risk of incident dementia, however engagement in these activities is constrained by the 24-hour day. Increasing time spent in one activity necessarily reduces time spent in another making it difficult to fully understand the implications of current behavioral modification recommendations. This study examines how reallocating time spent among sleep, sedentary behavior (SB), light physical activity (LPA), and moderate-to-vigorous physical activity (MVPA) within a 24-hour day impacts dementia risk in older adults. METHODS The compositional data analysis was conducted on data from 51,314 participants (mean [SD] age: 67.28 [4.30] years; 53.9% female) from the UK Biobank dataset. Participants wore wrist accelerometers for one week, and sleep, SB, LPA, and MVPA were measured using machine learning techniques. Over an average follow-up period of 6.61 (SD = 1.13) years, 410 participants were diagnosed with dementia. RESULTS Compositional data analysis using Cox proportional hazard models revealed that reallocating one hour per day to SB from all other behaviors was associated with a 114% increased risk of dementia (HR = 2.14, 95% CI: 1.43-3.22). Conversely, reallocating one hour per day to MVPA from other behaviors was associated with a 17% reduction in dementia risk (HR = 0.83, 95% CI: 0.76-0.91). Reallocations to sleep or LPA were not associated with changes in dementia risk unless they impacted time spent in SB or MVPA. CONCLUSIONS These findings highlight the critical importance of reducing sedentary time and increasing MVPA to lower dementia risk and suggest that interventions targeting these behaviors may be vital for brain health and dementia prevention.