The toll of an early bell? School start time and adolescent sleep quality in Flanders, Belgium

Study objectives: Adolescent sleep deprivation is a major public health concern, with insufficient and poor-quality sleep associated with adverse physical, mental and cognitive outcomes. Early school start times may contribute to sleep restriction by conflicting with the biologically delayed sleep patterns that emerge during puberty. This study examined the effect of school start times on sleep duration and sleep quality among secondary school students. Methods: Data were obtained from the 2022 Flemish Health Behaviour in School-aged Children (HBSC) study, including 12,329 adolescents from grade 1 to 6 in 94 secondary schools in Flanders. School start times were obtained from participating schools and sleep duration and sleep quality were assessed using self-report measures. Data were analyzed using traditional statistical methods (i.e., linear regression) and debiased machine learning methods (i.e., assumption lean regression, dose response curves and modified treatment policies). Results: No clinically meaningful effects of school start times on sleep outcomes were observed. In machine learning models, a 10-minute delay in start time was associated with non-significant changes in sleep quality (0.01 points on a 1-5 scale; 95% CI, -0.05 to 0.06) and school-day sleep duration (3.5 minutes; 95% CI, -1.0 to 8.0). Conclusions: Within the narrow range of school start times observed in Flemish secondary schools, confidence intervals excluded clinically meaningful effects of later start times on adolescent sleep duration and sleep quality. Whether larger delays would improve sleep remains to be determined.