Relations were examined between children's sleep and their externalizing and internalizing symptoms. Longitudinal relations were examined when children were in 3rd (T1) and 5th (T2) grades, and cross-sectional relations were assessed at T2. Participants included 176 children at T1 (M = 8.68 years) and 141 children at T2 (M = 10.70 years). Sleep was examined via subjective reports and actigraphy. Children reported on anxiety, self-esteem, and depression symptoms, and parents reported on children's externalizing and internalizing symptoms. Cross-sectionally and longitudinally, sleep problems were associated with worse adjustment outcomes; African American children or those from lower socioeconomic status homes were at particular risk. Findings highlight the importance of adequate sleep for children's optimal development, especially in the context of ecological risk.