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Background: The structured days hypothesis posits that 'structured days' (i.e., days with pre-planned, segmented, and adult-supervised environments) reduce youth obesogenic behaviors. Structured days may be especially important for adolescents', as adolescence (12-19 years) is a period of developmental milestones and increased autonomy. Therefore, the objective of this systematic review and meta-analysis is to evaluate the relationship between structured days and adolescents' obesogenic behaviors (i.e., physical activity, diet, screen time, and/or sleep)., Methods: From February to April of 2020, four databases (i.e., Embase, PubMed, Web of Science, and PsychINfo) were searched for cross-sectional, longitudinal, and intervention (i.e., baseline data only) studies reporting obesogenic behaviors on more structured versus less structured days (i.e., weekday versus weekend or school year versus summer/holiday)., Results: A total of 42,878 unique titles and abstracts were screened with 2767 full-text articles retrieved. After review of full-text articles, 296 studies were identified (sleep k = 147, physical activity k = 88, screen time k = 81, diet k = 8). Most studies were conducted in North America, Europe & Central Asia, or East Asia & the Pacific used self-report measures and compared school days to weekend days. Meta-analyses indicated that adolescents' physical activity (standardized mean difference [SMD] = -0.25 [95%CI - 0.48, -0.03]) and screen time (SMD = -0.48 [95%CI - 0.66, -0.29]) were less healthy on less structured days. Differences did not reach statistical significance for sleep (SMD = -0.23 [95%CI - 0.48, 0.02]) and diet (SMD = -0.13 [95%CI - 0.77, 0.51]), however, sleep timing (SMD = -1.05 [95%CI - 1.31, -0.79]) and diet quantity (SMD = -0.29 [95%CI - 0.35, -0.23]) were less healthy on less structured days. The review identified studies with large heterogeneity., Conclusions: Findings indicate that adolescents' physical activity, screen time, sleep timing, and diet quantity are less healthy on less structured days. Interventions for adolescents to prevent and treat obesity may be more successful if they are designed to target times that are less structured., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)