Abstract
BACKGROUND: This is the first study to examine longitudinal associations between self-selected timing of moderate-to-vigorous physical activity (MVPA) and health outcomes in young adults over 18 months. METHODS: Young adults (N = 434, Mage = 23.9, SDage = 4.6 years) enrolled in a weight management trial recorded 4-7 days of ActiGraph wear time at ≥1 time point (baseline, months 6, 12, and 18). Time-of-day categories were based on quartiles of the temporal distribution of MVPA min/h at baseline: morning (06:00-11:59), afternoon (12:00-15:59), evening (16:00-18:59), and night (19:00-00:59). The proportion of weekly MVPA accumulated during each time category was the predictor in longitudinal linear mixed-effects models predicting body mass index (BMI) and total weekly MVPA. Longitudinal quasibinomial generalized estimating equations models predicted cardiometabolic risk. Interactions were tested, and marginal trend estimates were generated for sex and age subgroups. RESULTS: The analytic sample was 79% female and 49% non-Hispanic White, with a mean (±SD) weekly MVPA of 311 ± 167 min at baseline. In adjusted models, there were no associations with BMI. Morning MVPA was inversely associated with cardiometabolic risk (OR [95% CI]: 0.99 [0.98-0.99]) for both sex and age groups. Evening MVPA was inversely associated with cardiometabolic risk for 26-35 year olds (0.98 [0.97-0.99]). Morning MVPA was associated with greater total MVPA across subgroups, and afternoon MVPA was associated with less total MVPA in women. CONCLUSIONS: Over 18 months, incremental health benefits may accrue with optimal activity timing in young adults. Activity-based interventions designed to improve cardiometabolic and behavioral health outcomes in young adults may be optimized by tailoring timing recommendations to demographic factors.