Project Summary Regular physical activity induces specific adaptive responses in various tissues, and ultimately improves health and reduces the risk for cardiovascular diseases and type 2 diabetes. To improve the efficacy of physical activity, many studies have examined the impacts of intensity, duration, frequency, and modality of physical activity on the adaptive responses. However, timing of physical activity has been rarely considered. It is well-established that the circadian timing system modulates cardiovascular function and metabolism, generating ~24-h rhythms in these physiological functions that are in synchrony with the day-night cycle. Thus, physical activity at different times of day may have different impacts on health due to the circadian control. Indeed, recent breakthrough animal studies have shown time-of-day dependent effects of exercise on transcripts and metabolites enriched in metabolic pathways that are related to exercise adaptations and, importantly, linked to the circadian system. We hypothesize that timing of physical activity modulates the adaptive responses, and can thus be used to enhance physical activity-induced health benefits. To address this hypothesis, during the K99 phase, I will first utilize the dataset from an NIH-funded randomized controlled trial with lifestyle intervention and 1-week accelerometer recording among approximately 2,200 patients with type 2 diabetes to examine 1) the association between timing of physical activity and markers of cardiometabolic health at baseline, and 2) the association between timing of physical activity and the improvements in markers of cardiometabolic health at 1-year follow-up. In the R00 phase, I will test the causal relationship by experimentally manipulating the timing of physical activity in individuals with prediabetes using a randomized, cross-over design with two highly controlled in-laboratory protocols. This innovative project will advance our knowledge in the interaction effects of the circadian timing system and physical activity and may help in designing evidence-based lifestyle interventions incorporating timing of physical activity. I am well suited to perform this research based on 1) my expertise in chronobiology, physiology, and human experimental research; 2) the exceptional multi-disciplinary mentoring team comprised of leaders in their respective fields; and 3) the unparalleled research environment to support my career development. Through this study, I will further my training in circadian physiology, as well as expand my expertise in exercise physiology, epidemiology, and clinical trial design. The proposed research and training will help achieve my long-term goal of launching an independent research program dedicated to understanding the interaction of the circadian system and physical activity from observational towards experimental study design. The findings of this research have the potential to frame the basic strategy and recommendation of chronobiology-based exercise interventions.