This cohort study examines how delaying school start time is associated with objectively assessed sleep duration, timing, and quality among adolescents from public high schools in Minnesota. Importance Sleep is a resource that has been associated with health and well-being; however, sleep insufficiency is common among adolescents. Objective To examine how delaying school start time is associated with objectively assessed sleep duration, timing, and quality in a cohort of adolescents. Design, Setting, and Participants This observational cohort study took advantage of district-initiated modifications in the starting times of 5 public high schools in the metropolitan area of Minneapolis and St Paul, Minnesota. A total of 455 students were followed up from grade 9 (May 3 to June 3, 2016) through grade 11 (March 15 to May 21, 2018). Data were analyzed from February 1 to July 24, 2019. Exposures All 5 participating schools started early (7:30amor 7:45am) at baseline (2016). At follow-up 1 (2017) and continuing through follow-up 2 (2018), 2 schools delayed their start times by 50 and 65 minutes, whereas 3 comparison schools started at 7:30amthroughout the observation period. Main Outcomes and Measures Wrist actigraphy was used to derive indices of sleep duration, timing, and quality. With a difference-in-difference design, linear mixed-effects models were used to estimate differences in changes in sleep time between delayed-start and comparison schools. Results A total of 455 students were included in the analysis (among those identifying sex, 225 girls [49.5%] and 219 boys [48.1%]; mean [SD] age at baseline, 15.2 [0.3] years). Relative to the change observed in the comparison schools, students who attended delayed-start schools had an additional mean 41 (95% CI, 25-57) objectively measured minutes of night sleep at follow-up 1 and 43 (95% CI, 25-61) at follow-up 2. Delayed start times were not associated with falling asleep later on school nights at follow-ups, and students attending these schools had a mean difference-in-differences change in weekend night sleep of -24 (95% CI, -51 to 2) minutes from baseline to follow-up 1 and -34 (95% CI, -65 to -3) minutes from baseline to follow-up 2, relative to comparison school participants. Differences in differences for school night sleep onset, weekend sleep onset latency, sleep midpoints, sleep efficiency, and the sleep fragmentation index between the 2 conditions were minimal. Conclusions and Relevance This study found that delaying high school start times could extend adolescent school night sleep duration and lessen their need for catch-up sleep on weekends. These findings suggest that later start times could be a durable strategy for addressing population-wide adolescent sleep deficits. Question How is a delay in high school start time associated with adolescent sleep? Findings In this cohort study of 455 high school students, those attending schools that shifted to later starts after baseline measurements (1) got approximately 43 minutes more objectively measured sleep on school nights, (2) slept less on weekends, and (3) had similar bedtimes 2 years after the start time delay, relative to students attending comparison schools that started early throughout the observation period. Meaning These findings suggest that delayed school start times may be a readily deployable sleep promotion intervention that can effectively allow adolescents greater opportunity for healthy sleep.
