BACKGROUND: In the US, children are at a greater risk of excessive weight gain during summer vacation compared to the school year. The Structured Days Hypothesis theorizes this is because in summer, children spend less time in structured environments that promote healthy behaviors. However, little is known about how children spend their time during the summer vs school and how this may differ by income and age. METHODS: In an observational cohort of 890 children (50% girls, 43% ≤200% Poverty, grades K-6th), parents completed a time use record (TUR) on their smartphones every evening for 14d in April/May (school) and 14d in July (summer) in 2021 and 2022. Parents reported the timing of setting-specific contexts of their child’s daily activities (e.g., afterschool program, sports, summer programs). Zero-inflated hurdle models estimated the probability of involvement in structure (0/1) and percent of wake time children spent in structured settings (non-zeros) between school and summer. Poverty status (living in poverty [LP] vs not living in poverty [NLP]) and grade were examined as moderators. RESULTS: A total of 17,029 TUR were completed. Compared to children NLP, children LP were more likely to never engage in structured activities outside of school (19% vs 38%, OR 0.19, 95CI 0.11-0.34) or during summer (23% vs 32%, 0.51, 0.28-0.92). For children NLP, involvement in structured activities during school peaked at 3rd grade and declined through 6th; while summer structure was greatest during K and declined through 6th. For children LP, involvement in structure did not vary across grades during school or summer. For children involved in structure, percent of time spent in structure varied by grade and income. During school and summer, children LP spent the greatest amount of time in structure in K and this declined thru 6th. Conversely, children NLP in 3rd and 4th spent the greatest amount of time in structure during school, whereas during summer children in K spent the greatest amount of time in structure. CONCLUSIONS: During school and summer, fewer children LP were involved in structured activities compared to children NLP. However, when they were involved, they spent a greater percentage of their day in structure compared to children NLP. Additional policies are needed to reduce the gap between children LP vs NLP and involvement in structured programs during school and summer. Grant Funding: Research reported in this publication was supported by the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health under Award Number R01DK116665. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

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