Background <p>Co-creation has proven effective in developing school-based healthy sleep interventions with and for adolescents. However, co-created interventions are context-specific. To increase scalability and broaden impact, co-created interventions might be adapted to new contexts via co-adaptation, defined as the collaborative tailoring of a (previously co-created) intervention to a new context. This study evaluates a co-adapted 9-week school-based healthy sleep intervention for adolescents in a three-arm pilot trial.</p> Methods <p>One school co-adapted a previously co-created intervention using Intervention Mapping. Another school implemented the original intervention with minor adjustments, while a third served as control and received no intervention. Data on sleep parameters, psychosocial determinants, and sleep-hygiene behaviours were collected via questionnaires at baseline (October 2022), post (May 2023), and follow-up (October 2023) (<i>n</i> = 170; 13.07 ± 0.71 years; 58.33% girls). Generalised Linear Mixed Models (GLMM) were fitted in R. Intervention acceptance was assessed in students from intervention schools through post-test questionnaires and four focus groups (<i>n</i> = 9, <i>n</i> = 2, <i>n</i> = 8, <i>n</i> = 4). Analyses were performed in MaxQDA.</p> Results <p>Both intervention schools showed favourable effects on daytime sleepiness at follow-up and objective weekday sleep duration at post versus the control. The co-adaptation school also showed favourable effects on knowledge at post. The co-adaptation school showed favourable effects on perceived sleep quality and mental wellbeing at follow-up, but unfavourable effects on stress, screen use at follow-up, and objective weekday sleep duration at post compared to the standard school. No significant effects emerged for other sleep parameters, psychosocial determinants, or sleep-hygiene behaviours. While students in both intervention schools noted incomplete delivery, the co-adaptation school reported better intervention acceptance (e.g., perception of intervention components and lesson- and class competition delivery).</p> Conclusion <p>Co-adaptation may enhance intervention acceptance but requires careful cost-benefit consideration, as the standard intervention yielded similar yet smaller effects. Further research with more schools per arm is needed.</p> Trial registration <p>This study is registered as clinical trial (NCT05838339). Registration date: 07.11.2022.</p>