Understanding associations between timing of physical activity and health outcomes in young adults - ABSTRACT To prevent and manage chronic disease, national guidelines call for a minimum of 150 minutes of moderate-to- vigorous physical activity per week; yet little is known about what time of day is best for performing this activity. The timing of physical activity may be particularly important for young adults who are vulnerable to social jetlag (i.e., discrepancy between biological rhythms and sleep/wake behaviors), placing them at increased risk of developing chronic diseases. Research in adult samples indicates that morning activity may be beneficial for weight management and/or cardiometabolic risk factors; however, there is a lack of longitudinal studies to support these findings. Furthermore, little is known about the timing of physical activity and health outcomes among young adults who are physiologically different from their adult counterparts and at increased risk of social jetlag/circadian misalignment. We aim to examine the cross-sectional and longitudinal relationships of physical activity timing with weight and cardiometabolic risk factors in a cohort of young adults at risk for cardiometabolic disease. Specifically, we will use data from the Healthy Body Healthy U (HBHU) trial [NIH/NIDDK R01 DK100916], a completed two-site weight management randomized controlled trial for young adults, ages 18-35. We will use objective physical activity data measured via the ActiGraph accelerometer at four time-points (Baseline, Month 6, Month 12, Month 18) to identify activity time categories (i.e., morning, afternoon, evening, night-time) and examine how differences at baseline, and changes over time, in activity during each time category relate to health outcomes (i.e., weight, cardiometabolic risk factors). We hypothesize that: 1) morning activity will be associated with lower weight at baseline and greater weight loss over the follow-up, 2) morning activity will be associated with fewer cardiometabolic risk factors at baseline and reductions in these risk factors over the follow-up, 3) the proposed associations between morning activity and improved weight/cardiometabolic risk factors will be mediated by greater weekly volumes of moderate-to- vigorous physical activity, and 4) age, sex, and intervention status will moderate the relationships. The proposed study is innovative in (a) its use of longitudinal data over 18 months to test associations between timing of activity and health outcomes, (b) its use of an at-risk young adult sample and (c) its use of a randomized controlled design to test the impact of a technology-based health messaging intervention on the associations of interest. The long-term goal of our research is to inform the design of timing-focused, theory- based behavior change interventions for at-risk young adults.
