Chronotype, or morningness/eveningness, has been associated with adjustment in both children and adolescents. Specifically, eveningness has been linked to adjustment difficulties; however, the mechanism underlying this association is poorly understood. The purpose of this study was to test whether the associations between eveningness and adjustment difficulties could be explained by an unfavorable impact of eveningness on sleep. Links from chronotype to internalizing problems and problem behaviors via sleep quantity and sleep problems were tested in a sample from the European Longitudinal Study of Pregnancy and Childhood (N = 3485; 48.8% female), both when the participants were children (7 years at T1, 11 at T2) and when they were adolescents (15 years at T1, 18 at T2). The findings provided evidence that eveningness predicted greater sleep problems and lower sleep quantity; however, only sleep problems predicted internalizing problems and problem behaviors. Sleep quantity did not mediate the eveningness-adjustment link, and sleep problems did so only in children. The findings show that sleep problems appear to be more important in explaining the eveningness-adjustment link rather than altered sleep quantity, commonly associated with eveningness.