Although numerous studies among adults have shown a U-shaped association between sleep duration and health outcomes, fewer studies have investigated the theory that children also have an optimal sleep duration range, with both lower and upper limits. We evaluated whether children's sleep duration at ages 5 and 9 has a U-shaped association with both behavioral problems and physical health at age 9. We analyzed data from 1,965 participants in a longitudinal birth cohort, the Fragile Families and Child Wellbeing Study. This sample of children was 52% male and approximately 22% non-Hispanic White, 52% non-Hispanic Black, 23% Hispanic, and 3% some other race/ethnicity. The child's primary caregiver reported the predictor of interest: sleep duration at age 5 and age 9. Both children and primary caregivers reported on outcomes of the child's behavior problems (internalizing and externalizing) and overall physical health. We found that the association between children's sleep duration and well-being was typically nonlinear and U-shaped. Adjusting for their sleep duration at age 5, children who sleep either too much or too little at age 9 had higher levels of behavior problems and scored lower on a global measure of physical health. These nonlinear patterns were similar whether children or primary caregivers reported child outcomes, with the exception that there was a linear and increasing association of longer sleep duration and caregiver-rated child health. This study highlights that both short and long sleep duration may be risk factors for adverse behavioral and health outcomes in school-age children.