Insufficient sleep and excess weight status contribute to adverse health outcomes across the lifespan, including risk for cardiometabolic disease. Cross-sectional data suggest that children with overweight/obesity are more likely to experience sleep disturbances than their non-overweight peers. Although the nature of this association may be bidirectional, prospective studies indicate that sleep impacts body weight regulation through multiple physiological and psychological pathways. In particular, insufficient sleep is related to greater energy intake and reduced diet quality in children. Although mechanisms explaining the association between sleep and eating behavior are poorly understood, sleep restriction has been found to impact brain processes related to reward valuation of food and self-regulation, the behavioral manifestations of which may increase susceptibility to suboptimal dietary behaviors and subsequent weight gain. A limitation of prior research on mechanisms is that much of it has been conducted in adults and in laboratory settings, thereby calling into question the ecological validity of the findings. Alternatively, studies on sleep restriction/extension in children's natural environments have relied on retrospective reporting of eating behavior, included children across the weight spectrum, and had limited focus on underlying mechanisms, particularly neural substrates. A clearer understanding of momentary mechanisms involved in the sleep/eating association could improve development and/or refinement of sleep-related interventions, particularly those delivered in real time when risk for engaging in maladaptive eating is highest. The proposed R01 study will examine prospective associations among sleep, eating-related self-regulation, and eating behavior in the natural environment. Community-based children with overweight or obesity (n=120) will undergo a naturalistic protocol involving assessment of typical sleep and eating patterns (week 1), followed by sleep restriction or extension (weeks 2 and 3, separated by a 7-day wash-out), the latter occurring within a randomized crossover design. Assessment throughout the study period will involve daily actigraphy measurement of sleep patterns; repeated daily self-reports on eating behavior and behavioral assessment of eating-related self-regulation; and intermittent 24-hour dietary recalls informed by daily real-time food photography. Participants will complete fMRI-based assessment of neural activation during an eating-related self-regulation task after each week-long period of sleep restriction and extension. Overall aims are to assess short-term effects of sleep extension versus restriction on eating-related self-regulation (including behavioral and neural performance) and naturalistic eating behavior. These data will clarify timing and trajectory of changes in eating behavior and self-regulatory mechanisms as a consequence of sleep patterns. The proposed study has clear potential to advance scientific and clinical understanding of mechanisms involved in the prospective associations between inadequate sleep and maladaptive eating in youth and inform interventions to alleviate their cumulative personal and societal burden.