This article summarizes correlational, case-control, quasi-experimental, and experimental studies that have examined whether sleep during childhood and adolescence is related to daytime functioning. Published findings suggest that inadequate sleep quality and/or quantity can cause sleepiness, inattention and, very likely, other cognitive and behavioral deficits that significantly impact children and adolescents in functional settings. This article then integrates findings from longitudinal studies within a developmental psychopathology model. Important questions remain, but evidence supports the integration of sleep screening and interventions into routine clinical care and also supports advocacy for public policy changes to improve the sleep of children and adolescents.