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substance use (caffeine, alcohol, and nicotine), and depressive symptoms (lockdown sample only; assessed using the withdrawn/depressed scale from the Youth Self Report).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

Caffeine is the most commonly ingested psychoactive drug worldwide with increasing consumption rates among young individuals. While caffeine leads to decreased sleep quality in adults, studies investigating how caffeine consumption affects children’s and adolescents’ sleep remain scarce. We explored the effects of regular caffeine consumption on sleep behavior and the sleep electroencephalogram (EEG) in children and adolescents (10–16 years). While later habitual bedtimes (Caffeine 23:14 ± 11.4, Controls 22:17 ± 15.4) and less time in bed were found in caffeine consumers compared to the control group (Caffeine 08:10 ± 13.3, Controls 09:03 ± 16.1), morning tiredness was unaffected. Furthermore, caffeine consumers exhibited reduced sleep EEG slow-wave activity (SWA, 1–4.5 Hz) at the beginning of the night compared to controls (20% ± 9% average reduction across all electrodes and subjects). Comparable reductions were found for alpha activity (8.25–9.75 Hz). These effects, however, disappeared in the morning hours. Our findings suggest that caffeine consumption in adolescents may lead to later bedtimes and reduced SWA, a well-established marker of sleep depth. Because deep sleep is involved in recovery processes during sleep, further research is needed to understand whether a caffeine-induced loss of sleep depth interacts with neuronal network refinement processes that occur during the sensitive period of adolescent development.