Relations between children’s sleep and cognitive functioning were examined over 2 years, and race and socioeconomic status were assessed as moderators of effects. Third-grade African American and European American children (N = 166; M = 8.72 years) participated at Time 1 and again 2 years later (N = 132). At both Time 1 and Time 2, sleep was examined via self-report and actigraphy. Children were administered selected tests from the Woodcock–Johnson III Tests of Cognitive Abilities, and Stanford Achievement Test scores were obtained from schools. Children’s sleep was related to intellectual ability and academic achievement. Results build substantially on an emerging literature supportive of the importance of sleep in children.