School start time influences melatonin and cortisol levels in children and adolescents – a community-based study

ABSTRACT

School start time influences sleep parameters. Differences between circadian sleep parameters on weekends and weekdays have been associated with obesity, sleep, and psychiatric disorders. Moreover, circadian rhythm dysregulation affects the secretion of some hormones, such as melatonin and cortisol. In the current study, we investigate the effect of school start time on cortisol and melatonin levels in a community sample of Brazilian children and adolescents. This was a cross-sectional study of 454 students (mean age, 12.81 ± 2.56 years; 58.6% female). From this sample, 80 participants were randomly selected for saliva collection to measure melatonin and cortisol levels. Circadian sleep parameters were assessed by self-reported sleep and wake up schedules and the Morningness–Eveningness Questionnaire. The outcomes, salivary melatonin and cortisol levels, were measured in morning, afternoon and night saliva samples, and behavior problems were assessed using the Child Behavior Checklist (CBCL). The main results revealed that morning school start time decreased the secretion of melatonin. Morning melatonin levels were significantly positively correlated with the sleep midpoint on weekdays and on weekends. Afternoon melatonin levels were positively correlated with the sleep midpoint on weekends in the morning school students. Conversely, in the afternoon school students, night melatonin levels were negatively correlated with the sleep midpoint on weekdays. Cortisol secretion did not correlate with circadian sleep parameters in any of the school time groups. In conclusion, school start time influences melatonin secretion, which correlated with circadian sleep parameters. This correlation depends on the presence of psychiatric symptoms. Our findings emphasize the importance of drawing attention to the influence of school start time on the circadian rhythm of children and adolescents.

KEYWORDS:

• Behavior problems
• child behavior check list
• circadian rhythm
• hormones
• school start time

Acknowledgments

We thank the Escola Estadual de Educação Básica São Francisco, Escola São Valentim, Colégio Madre Bárbara, and the teachers of these schools for their assistance in data collection.

Declaration of interest

Authors have reported no financial conflicts of interest regarding the subject of the present study. Funding source: This study was partly supported by the Fundo de Incentivo à Pesquisa (FIPE), Hospital de Clínicas de Porto Alegre (HCPA, Brazil), project number 12-0386. AC and FD received grants from the Brazilian government (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior; CAPES). ACM received a science popularization scholarship (Bolsa de Iniciação à Popularização da Ciência; BIPOP) from the Pró-Reitoria de Pesquisa (PROPESQ), Universidade Federal do Rio Grande do Sul. MPH received funding from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).