Pre-registration: Effects of school start times on adolescent sleep in Stockholm high schools

The study uses naturally occurring variation in school start times across the classes, those with delayed SST and a selection of classes that maintained their original schedules, treating SST as a continuous class-level exposure variable. Data were collected from n=527 upper secondary students (gymnasium, year 2) across 20 classes in three schools in Stockholm. Questionnaire data were collected at four time points: at the start of the school term (baseline), and at 1, 3, and 6 months follow-up.

The primary outcome for this pre-registration is weekday sleep duration, measured via self-report using the Karolinska Sleep Questionnaire (KSQ). The exposure variable is the weekly average school start time per class, derived from class schedules, expressed in hours from midnight and centered at 8:00 a.m. The study uses a Bayesian multilevel modelling approach (three-level: measurement occasions nested within students nested within classes), implemented in R using the brms package.

Missing data are handled via model-based imputation in brms.