Background: This study evaluated the effect of the school-based intervention Charge Your Brainzzz on adolescents' social-cognitive determinants, sleep hygiene and sleep duration and quality. Methods: A cluster-randomized controlled trial was conducted with 972 students from 10 Dutch high schools. Schools were randomly allocated to the intervention (N = 5) or control condition (N = 5). Outcomes were measured with the digital Consensus Sleep Diary and via a digital questionnaire, based on valid measures. Data were collected at baseline (T0), ±1.5 weeks post-intervention (T1) and ±3 months post-intervention (T2). Mixed model analyses were performed to estimate the effects on social-cognitive determinants, sleep hygiene, and sleep outcomes. Results: The intervention increased sleep knowledge post-intervention (b = 1.91; 95%CI: 1.22-2.60) and at follow up (b = 1.40; 95%CI: 0.70-2.10). The intervention was also effective in changing adolescents' attitudes (b = 0.10; 95%CI: 0.01-0.19) and perceived behavioral control (b = 0.11; 95%CI: 0.01-0.22) post-intervention. No positive changes were found regarding subjective norms, behavioral intentions, sleep hygiene, or sleep outcomes. Conclusions: The intervention improved adolescents' sleep knowledge, attitude, and perceived behavioral control. To significantly impact sleep health, theoretically sound and systematically developed interventions are needed which take into account the interplay between sleep, sleep-related behaviors, and adolescents' social and physical environment.