OBJECTIVE: Sleep deprivation and reduced sleep quality are common in adolescents and negatively impact their physical and mental wellbeing. This study evaluates the effect of a participatory-developed school-based healthy sleep intervention for adolescents. METHOD: A 16-week long intervention, cocreated with adolescents, was conducted with two schools with four schools serving as measurement-only controls. Intervention elements included a kickoff event, posters, Instagram posts, an application, and class activities. Data on sleep parameters, sleep hygiene, and psychosocial factors were collected before, immediately after, and 6 months after implementation ( RESULTS: Participants in the intervention group demonstrated significant increases in sleep knowledge and larger decreases in supportive peer and parental factors compared to the control group. At 6 months, the intervention group displayed increased self-reported weekend sleep duration and objectively measured week sleep duration. There was a significantly smaller increase in the use of screens in bed and barriers toward screen use in bed in the intervention group compared to the control group. Finally, a significant decrease in peer modeling and mental support of parents was observed in the intervention group (versus significant increase and no significant change in the control group, respectively). CONCLUSION: Researchers developing healthy sleep interventions should consider combining a participatory approach with a theory-based protocol including the use of active parental components. (PsycInfo Database Record (c) 2025 APA, all rights reserved).