Effects of reallocating physical activity, sedentary behaviors, and sleep on mental health in adolescents

Abstract Objective Physical activity, sedentary behavior, and sleep are associated with mental health in adolescents. Mental health may depend not only on the amount of time spent in a specific activity, but also on the activity it displaces. The aim of this study was to examine the impact of reallocating 15 min of time spent in one health behavior with 15 min in another on adolescent mental health. Methods Cross-sectional data from the students participating in the COMPASS Study (2018–2019) were analyzed (N = 46,413). Participants self-reported the amount of time they spent engaged in moderate-to-vigorous physical activity (MVPA), doing homework and using screens, and their sleep duration on average each day, depressive symptoms, anxiety, and flourishing. Data were analyzed using isotemporal substitution modeling. Results Among adolescents getting less than the recommended amount of sleep, replacing any behavior with sleep was generally associated with better mental health outcomes. Conversely, among adolescents getting adequate sleep, the findings did not support replacing other behaviors with sleep with the exception of screen time. Replacing homework and MVPA with sleep was associated with less flourishing regardless of sleep duration. Conclusions Replacing screen time with any behavior may be better for mental health outcomes. Results provide further support for the critical role of sleep in promoting healthy development during adolescence, though more sleep than is recommended may confer little benefit for mental health. The findings demonstrate that mental health benefits may be obtainable at intervals as short as 15 min.