IMPORTANCEAlthough negative associations of COVID-19 pandemic high school closures with adolescents' health have been demonstrated repeatedly, some research has reported a beneficial association of these closures with adolescents' sleep. The present study was, to our knowledge, the first to combine both perspectives. OBJECTIVE To investigate associations between adolescents' sleep and health-related characteristics during COVID-19 pandemic school closures in Switzerland. DESIGN, SETTING, AND PARTICIPANTS This survey study used cross-sectional online surveys circulated among the students of 21 public high schools in Zurich, Switzerland. The control sample completed the survey under regular, prepandemic conditions (May to July 2017) and the lockdown sample during school closures (May to June 2020). Survey respondents were included in the study if they provided their sex, age, and school. EXPOSURES High school closures during the first COVID-19 pandemic wave in Switzerland (March 13 to June 6, 2020). MAIN OUTCOMES AND MEASURES Sleep-wake patterns, health-related quality of life (HRQoL, assessed by the KIDSCREEN-10 questionnaire), substance use (caffeine, alcohol, and nicotine), and depressive symptoms (lockdown sample only; assessed using the withdrawn/depressed scale from the Youth Self Report). Multilevel regression models were used to assess sample differences and associations of health-related characteristics with sleep duration and depressive symptoms. RESULTS The total sample consisted of 8972 students, including 5308 (59.2%) in the control sample (3454 [65.1%] female) and 3664 (40.8%) in the lockdown sample (2429 [66.3%] female); the median age in both samples was 16 years (IQR, 15-17 years). During school closures, the sleep period on scheduled days was 75 minutes longer (semipartial R 2 statistic [R 2β* ], 0.238; 95% CI, 0.222-0.254; P < .001) and the students had better HRQoL (R 2 β* , 0.007; 95% CI, 0.004-0.012; P < .001) and less consumption of caffeine (R 2 β* , 0.010; 95% CI, 0.006-0.015; P < .001) and alcohol (R 2 β* , 0.014; 95% CI, 0.008-0.022; P < .001). Longer sleep duration was associated with better HRQoL (R 2 β* , 0.027; 95% CI, 0.020-0.034; P < .001) and less caffeine consumption (R 2 β* , 0.013; 95% CI, 0.009-0.019; P < .001). In the lockdown sample, an inverse association was found between depressive symptoms and HRQoL (R 2 β* , 0.285; 95% CI, 0.260-.0311; P < .001) and a positive association was found with caffeine consumption (R 2 β* , 0.003; 95% CI, 0.000-0.008; P = .01). CONCLUSIONS AND RELEVANCEIn this survey study, 2 opposing associations between school closures and adolescents' health were identified: a negative association with psychological distress and a beneficial association with increased sleep duration. These findings should be considered when (continued) Key Points Question Were sleep gains among adolescents during COVID-19 pandemic high school closures associated with better health-related characteristics? Findings In this survey study of 8972 adolescents from Swiss high scho...

Summary As the chronotype delays progressively throughout puberty, early morning school start times (SSTs) contradict the sleep biology of adolescents. Various studies have demonstrated beneficial effects of later SSTs on sleep and health; however, adolescents’ preferences for SSTs have to date never been investigated in detail. The present online survey study aimed to fill this gap and explored influencing factors. A total of 17 high schools in the Canton of Zurich, Switzerland, circulated the survey among their students. Participants were included if they reported their sex, age, and school (n = 5,308). Students indicated whether they preferred later SSTs. Additionally, five predictor blocks were assessed: sociodemographic, school‐related, sleep, leisure‐time, and health‐related characteristics. We applied multivariate logistic regression models with fixed and random effects to predict the preference. The mean (SD) age of the students was 16.09 (1.76) years (65.1% female). The majority (63.2%) endorsed later SSTs with a preferred delay of 55 min (interquartile range 25–75 min). In the multilevel analysis (n = 2,627), sex, mother tongue, sleep characteristics, mobile device use at bedtime, caffeine consumption, and health‐related quality of life were significant predictors for the preference. Hence, the majority of adolescents preferred later SSTs, and especially those with sleep or health‐related problems. These characteristics have been consistently shown to improve after delaying SSTs. Thus, also from adolescents’ view, later SSTs should be considered to improve the adolescents’ health.