The aim of this study was to examine the cross-sectional and longitudinal relationships among variables related to sleep patterns and both social-emotional problems (i.e., internalizing, externalizing, and dysregulation) and healthy social development (i.e., social competence). Assessments were completed at 6, 12, and 18 months across 5 cohorts of children for a total of 117 mother-child dyads. Mothers completed the Brief Infant Sleep Questionnaire at 6, 12, and 18 months, as well as the Infant Toddler Social Emotional Assessment at 12 and 18 months. Later bedtimes and less total sleep across the 24-hr period predicted higher internalizing problem scores, which includes indices of depression/withdrawal, general anxiety, separation distress, and inhibition. In contrast, sleep fragmentation was minimally associated with decreased social competence but not with any negative social-emotional outcomes. These results indicate that sleep patterns, primarily later bedtimes and less total sleep, appear to be associated with and predictive of social-emotional problem areas, namely, internalizing issues, in infants and toddlers. These findings add to the growing literature on the role of sleep in early social-emotional development and suggest that sleep schedule and duration should be addressed in clinical assessment and interventions for infant sleep.