Pre-registration: Effects of school start times on adolescent sleep in Stockholm high schools
https://doi.org/10.48723/w422-jb60
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.
Documentation files
Documentation files
Citation and access
Citation and access
Data access level:
Creator/Principal investigator(s):
Research principal:
Principal's reference number:
- 4-1426/2025
Data contains personal data:
Yes
Type of personal data:
Data contain pseudonymized identifiers. A code key exists for the data, which links the identifiers to participant's phone numbers. Information on schools and classes has been pseudonymized.
Code key exists:
Yes
Citation:
Method and outcome
Method and outcome
Unit of analysis:
Population:
The pre-registration uses simulated data, which have been created in a way to reflect the study population of the study, which consists of high-school students from a sample of 3 schools (20 classes) in Stockholm.
Time method:
Study design:
- Observational study
- Cohort study: Prospective
- Cohort study
Description of study design:
For the purpose of pre-registration, the files provided at this stage contain simulated data only. The simulated data has been created in a way that mirrors the study design and collection of the actual data (see description below).
This is an observational cohort study involving students from 20 classes across three upper secondary schools in Stockholm. Stockholm City decided to implement delayed school start times in selected classes for the 2025/26 academic year, meaning some classes start no earlier than 9:00 a.m. while other classes continue with their usual schedules. This created natural variation in school start times across classes, enabling an observational evaluation of how school start times affect student sleep. School start time is treated as a continuous class-level exposure variable rather than a binary intervention. Data were collected via digital self-report questionnaires at four time points during the 2025/26 academic year: baseline, and 1, 3, and 6 months follow-up.
Sampling procedure:
Description of sampling:
For the purpose of pre-registration, the files provided at this stage contain simulated data only. The simulated data has been created in a way that mirrors the study design and collection of the actual data (see description below).
The study was carried out at three Stockholm high-schools. These three schools were selected by the City of Stockholm's Education Administration. A total of 20 classes at the participating schools were sampled. Ten classes were chosen to implement school start times no earlier than 9:00 a.m., whereas ten classes were chosen by the schools to serve as control classes and to continue with their ordinary schedules. All students in the participating 20 classes were invited to participate in the surveys.
Time period(s) investigated:
Variables:
14
Data format/data structure:
Data collection - Self-administered questionnaire: Web-based (CAWI)
Data collection - Self-administered questionnaire: Web-based (CAWI)
Mode of collection:
Self-administered questionnaire: Web-based (CAWI)
Description of the mode of collection:
Participating students filled in a web-based self-report questionnaire at four time-points during the academic year 2025/26, i.e., at baseline, 1-month, 3-month, and 6-month follow-up.
Information on each classes school start times was retrieved from the classes schedules, provided by the school administrators.
Time period(s) for data collection:
2025-08 - 2026-07
Data collector:
- Karolinska Institutet
Opens a new window at ror.org.
ROR
Instrument
Instrument
Name:
Karolinska Sleep Questionnaire (KSQ)
Type:
Structured questionnaire
Description of the instrument:
The Karolinska Sleep Questionnaire (KSQ) is a validated questionnaire that measures sleep habits and sleep-related symptoms.
Geographic coverage
Geographic coverage
Geographic location:
Lowest geographic unit:
Municipality
Highest geographic unit:
Country
Administrative information
Administrative information
Responsible department/unit:
Department of Learning, Informatics, Management and Ethics [C7]
Commissioning organisation:
- Region Stockholm, Stockholm Health Care Services
Ethical Review
Ethical Review
Reviewer:
- Swedish Ethical Review Authority
components.catalogue.resource.content.administrativeInformation.ethicalReview.rorId.srText
ROR
Registration number:
2025-05095-02
Ethical review information:
Ethical approval was granted.
Ethical approval: 2025-03201-01
Approval of amendment: 2025-05095-02
