STUDY OBJECTIVES: Adequate sleep is essential for adolescents' physical, emotional, and cognitive well-being. However, accurately capturing the complex components of sleep in this demographic is challenging, especially with retrospective self-report measures. This study aims to compare sleep data obtained from youth reports, caregiver reports, and Fitbit devices among early adolescents. METHODS: Data from 11,879 adolescents (11-14 years, 47.83% female), in Year 2 of the Adolescent Brain Cognitive Development (ABCD) Study, were analyzed. Adolescents self-reported their sleep characteristics using the Munich Chronotype Questionnaire, and caregivers provided data through the Children's Sleep Disturbance Scale. Additionally, a subset of participants (N = 3,803) wore Fitbit Charge 2 devices for 21 days. We assessed the questionnaires' internal consistency and utilized Bland-Altman and interclass correlation analyses for comparing self-reported sleep characteristics with Fitbit measures. RESULTS: Youth-reported and caregiver-reported sleep questionnaires demonstrated acceptable internal consistency. Discrepancies between caregiver and adolescent reports were more pronounced when adolescents reported sleep periods of less than 7 hours. Compared to Fitbit measurements, adolescents' self-reports showed a reasonably high agreement on sleep period and bedtime, while agreement on duration of wakefulness after sleep onset and sleep onset latency was poor. CONCLUSIONS: Results show reasonable agreement between adolescent self-reports and Fitbit measurements of sleep period and indicate their usefulness in assessing sleep behavior in adolescents. Caregivers provided valuable perspectives of the youth's sleep disturbances, however, they tended to overestimate sleep duration. These findings offer important methodological insights and highlight the necessity of adopting multi-dimensional approaches to assess sleep in adolescents.