https://researchdata.se/en/catalogue Search results en For the purpose of pre-registration, the files provided at this stage contain simulated data only. The simulated data mirror the structure, variables, formats, and expected distributions of the real data that will be analysed. The simulation is grounded in observed parameters from the class schedules (School start times.csv) and published estimates of sleep duration in Swedish adolescents (Lemke et al., 2023). This dataset is connected to a longitudinal observational study examining the association between school start times (SST) and sleep in upper secondary students in Stockholm, Sweden. Stockholm City has implemented delayed school start times (no earlier than 9:00 a.m.) in selected classes across three upper secondary schools during the 2025/26 academic year. This created variation in start times across classes, enabling an observational evaluation of how SST affects adolescents' sleep. 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. Fri, 04 Sep 2026 12:03:20 GMT https://researchdata.se/en/catalogue/dataset/2026-240 https://researchdata.se/en/catalogue/dataset/2026-240 Karolinska Institutet Theresa Lemke Gergö Hadlaczky