Race and socioeconomic status (SES) moderated the link between children's sleep and cognitive functioning. One hundred and sixty-six 8- to 9-year-old African and European American children varying in SES participated. Sleep measures were actigraphy, sleep diaries, and self-report; cognitive measures were from the Woodcock–Johnson III and reaction time tasks. Children had similar performance when sleep was more optimal, but after controlling for SES, African American children had lower performance with sleep disruptions. Children from lower and higher SES had similar performance with better sleep quality and less variability in sleep schedules, but when sleep was more disrupted, higher SES children had better performance. Examination of environmental variables associated with race and SES that may underlie these effects may lead to directions for interventions to improve cognitive performance.