Substitutions of Sleep, Physical Activity, and Sedentary Behaviour and the Risk of Dementia and Stroke.

Sleep, sedentary behaviour, and physical activity (PA) are important for brain health. Spending more time in one behaviour always substitutes time in another, which may affect associations and should be considered in prevention strategies. We assessed how substitutions of sleep, sedentary behaviour, and PA are associated with incident dementia and stroke, using compositional analysis. Participants (mean age: 71.3 ± 9.26 years, 51.6% female) without prevalent dementia (N = 1899) or stroke (N = 1854) from the Rotterdam Study wore an accelerometer for ≥ 4 days to estimate the duration of sleep, sedentary behaviour, light PA, and moderate-to-vigorous PA. Participants were continuously followed up for dementia (median: 4.5 years) and stroke (median: 5.1 years). Compositional Cox regression with isotemporal substitution analysis was used to assess associations of 30-min pair-wise substitutions with dementia and stroke. In total, 50 (2.6%) participants were diagnosed with dementia and 75 (4.0%) with a first-time stroke. Spending more time in moderate-to-vigorous PA and less in other behaviours was associated with a lower risk of dementia (Hazard Ratio [HR] for 30 min less sedentary behaviour 0.36; 95% CI: 0.24-0.55) and so was more sleep and less sedentary behaviour (HR: 0.87; 0.79-0.97) or light PA (HR: 0.43; 0.27-0.68). Those with more light PA and less sedentary behaviour had a higher risk of dementia (HR: 1.78; 1.19-2.66). Only having more sleep and less sedentary behaviour was associated with having a stroke (HR: 1.14; 1.03-1.27). More time in sleep and moderate-to-vigorous PA, substituting particularly sedentary behaviour, may be a modifiable risk factor for dementia. No consistent effects for stroke were found, warranting future research.