Adolescents frequently experience sleep disturbances due to physiological and psychosocial changes. These disturbances are linked to impaired cognitive performance, emotional dysregulation, and mental health issues. Although cognitive behavioral therapy (CBT) interventions have shown efficacy in treating sleep disorders, few studies have implemented interventions in school settings nor investigated their impact on objective cognitive outcomes. This study evaluated the effectiveness of a school-compatible, workshop-based intervention partly based on CBT for insomnia and for DSPS in improving sleep parameters, cognitive performance, and mental health among adolescents. In a longitudinal experimental design, 100 adolescents (aged 15 to 25 years old) with selfreported sleep problems were randomly assigned to an immediate intervention group (experimental group) or a control group, which waited one month before receiving the intervention (waiting group). The intervention consisted of four weekly group workshops focusing on psychoeducation, behavioral strategies, and personalized sleep hygiene advice. Outcomes were measured using subjective (questionnaires, sleep diaries) and objective (SART, MIST, memory recall, actigraphy, melatonin levels) tools across three timepoints (T1, T2 and T3). Participants who received the intervention showed significant improvements in sleep efficiency, subjective sleep quality, daytime vigilance, declarative memory, and anxiety. Objective vigilance and physiological stress responses did not significantly change. The intervention seemed effective across different profiles, including two identified chronotypes. Actigraphy and salivary melatonin levels data has yet to be analyzed. This intervention combining CBT-I and CBT-DSPS, delivered in an accessible school-based format, led to meaningful improvements in sleep, cognitive and mental health outcomes.