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Sleep plays a foundational role in adolescent development, supporting emotional regulation, cognition and brain maturation. However, adolescents today face increasing challenges to healthy sleep partly due to widespread social media use and heightened reward sensitivity. While each factor has been studied independently, less is known about how they interact, especially whether brain activation shapes how social media influences sleep. We examined prospective associations between self-reported sleep duration, social media use and brain activation in 1985 adolescents (mean age, Year 2 = 11.91 years) from the Adolescent Brain Cognitive Development (ABCD) Study. Sleep duration was measured via the Munich Chronotype Questionnaire; social media use via the Youth Screen Time Survey and brain activity using fMRI during the Monetary Incentive Delay task. We tested three sets of prospective models to examine how sleep, social media use, executive control and reward-related brain activation influence each other over two years, controlling sociodemographics. Greater social media use at Year 2 predicted shorter sleep duration at Year 4, particularly among adolescents with lower activation in the nucleus accumbens, cingulate gyrus, insula and putamen. Shorter sleep also predicted increased social media use two years later, with effects moderated by activation in the insula and middle frontal gyrus. Longer sleep at Year 2 predicted higher caudate activation at Year 4. Brain activation in reward and executive control-related regions moderated associations between sleep and social media use, suggesting that lower neural engagement may reflect increased susceptibility to the sleep-disrupting effects of social media.