AbstractObjective: Moderate-to-vigorous physical activity (MVPA) is obesity-protective.However, the optimal time of the day to engage in MVPA for weight management iscontroversial. This study is designed to investigate the influence of the diurnal pattern of MVPA on the association between MVPA and obesity.Methods: A total of 5285 participants in the 2003 to 2006 National Health andNutrition Examination Survey (NHANES) were cross-sectionally analyzed. The diurnalpattern of objectively measured MVPA was classified into three clusters by K-meansclustering analysis: morning (n = 642); midday (n = 2456); and evening (n = 2187).The associations of MVPA level and the diurnal pattern with obesity were tested.Results: A strong linear association between MVPA and obesity was found in themorning group, whereas a weaker curvilinear association between MVPA and obesitywas observed in the midday and evening groups, respectively. Among those who metthe physical activity guidelines, the adjusted means for BMI were 25.9 (95% CI:25.2–26.6), 27.6 (95% CI: 27.1–28.1), and 27.2 (95% CI: 26.8–27.7) kg/m2 in themorning, midday, and evening groups, respectively, and for waist circumference were91.5 (95% CI: 89.4–93.6), 95.8 (95% CI: 94.7–96.9), and 95.0 (95% CI: 93.9–96.1)cm, respectively.Conclusions: The diurnal pattern of MVPA influences the association betweenMVPA and obesity. The promising role of morning MVPA for weight managementwarrants further investigation.