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substance use (caffeine, alcohol, and nicotine), and depressive symptoms (lockdown sample only; assessed using the withdrawn/depressed scale from the Youth Self Report).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...

Two opposing effects on adolescents’ health during COVID-19 lockdown have been described: A beneficial one due to longer sleep times during school closures and a detrimental one of psychological distress. This study investigated how sleep and health changed in the course of the pandemic when schools were open again. Overall, 12,238 adolescents in Switzerland participated in three cross-sectional online surveys: In 2017 under regular conditions (control group), during pandemic school closures in 2020 (closure group), and in 2021 still under pandemic conditions, but schools were open again (postclosure group). Sleep behavior and health-related characteristics (health-related quality of life; caffeine, alcohol, and nicotine use) in all three groups and depressive symptoms in the closure and postclosure groups were assessed. The sleep period on school days was longer in the closure group (median 9.00 h, interquartile range 8.25–9.75) and similar in the postclosure (7.92, 7.00–8.50) compared to the control group (7.75, 7.08–8.33). Health-related characteristics were better during school closures and similar to worse in the postclosure compared to the control group. Depressive symptom levels were higher in the postclosure than in the closure group. Therefore, beneficial effects were specific to school closures and adolescents’ psychological distress increased over the course of the pandemic.