Publication date: May 31, 2025

Although there is consistent evidence of unhealthy changes in the 24-h movement behaviours when comparing pre-COVID-19 periods to the early stages of the pandemic, there is limited research on long-term changes among adolescents. This study aimed to analyze both between- and within-participant differences in accelerometer-assessed 24-h movement behaviours by comparing cross-sectional and prospective data from the pre-COVID-19 period (August to December 2019) to the period following the reopening of schools for in-person classes in southern Brazil (August to December 2022). This is a repeated cross-sectional design with a nested cohort. The 24-h movement behaviours (i. e., light physical activity [LIPA] and moderate-to-vigorous physical activity [MVPA], sedentary behaviour [SB], and sleep time [SPT]) were assessed by processing raw accelerometer data derived from a 24-h/7-day wrist-worn protocol. Compositional multilevel models were applied to compare the 24-h movement behaviour composition between time points for both cross-sectional and prospective data. Self-reported sociodemographic characteristics were examined as potential moderators. The cross-sectional and prospective samples comprised, respectively, 1276 (53% female, average age of 16. 4 +/- 1. 1) and 249 (53% female, average age of 15. 6 +/- 0. 8) participants. The 24-h movement behaviour composition differed between time-points in the cross-sectional (p 

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