Patte, Karen A.Qian, WeiCole, Adam G.Faulkner, GuyChaput, Jean-PhilippeCarson, ValerieLeatherdale, Scott T.2018-12-132019-10-122018-12-132018-10-12https://doi.org/10.1016/j.sleep.2018.09.020http://hdl.handle.net/10012/14240The final publication is available at Elsevier via https://doi.org/10.1016/j.sleep.2018.09.020� 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Background To date, no longitudinal population-based studies of school start times have been conducted within Canada. School schedule changes provided an opportunity to examine start times in association with youth sleep, physical activity, and screen use over time. Methods This longitudinal study included grade 9�12 students attending 49 Ontario secondary schools that participated in at least two consecutive years of the COMPASS study (2012�2017). Fixed effects models tested whether differences in within-student change in self-reported sleep duration, moderate-to-vigorous physical activity, and screen time were associated with school start time changes, adjusting for student- (grade, sex, ethnicity, spending money) and school-level covariates (median income, urbanicity, geographical area). Results Thirteen start time changes of 5�10�min were reported. Ten-minute advances at earlier clock times (8:30 AM�8:20 AM; 8:40 AM�8:30 AM) were associated with steeper sleep duration declines than schools with consistent start times but had no effect at later times (9:00 AM�8:50 AM). While sleep change did not differ with 5-min�delays, 10-min�delays (8:50 AM�9:00 AM) were associated with additional sleep (23.7�min). Apart from one school that shifted from 8:30 AM to 8:35 AM, in which screen time and physical activity decreased more steeply, no effect was found for screen time, and 5-min�delays were associated with more physical activity (10.9�min) and advances with less activity (_8.0�min). Conclusions Results support start time delays as a valuable strategy to help ameliorate sleep debt among youth. Interference with physical activity or increased screen time appear unlikely with modest schedule changes. Potential adverse impacts on sleep require consideration with 10-min�advances.enAttribution-NonCommercial-NoDerivatives 4.0 Internationaladolescentsphysical activityschool start timescreen timesleep durationyouthArticle