Evidence that sleep influences social and cognitive adaptation for school-age children and adolescents is accumulating rapidly, but less research focuses on the role of sleep for adaptive functioning during early childhood. We addressed these questions using actigraphy to assess sleep duration, sleep quality, and variability in sleep schedules in relation to a range of social/emotional and cognitive measures, including receptive vocabulary, emotion understanding, peer acceptance, social skills, social engagement, and temperament. Children in a convenience sample (N = 62, 40 boys, mean age = 4.15 yrs, 67% European American) wore actigraphs for 4–7 days, with sleep and wake states determined using Sadeh's scoring algorithm. Older children spent less time in bed at night and ethnic minority children (mostly African Americans) slept less at night and had lower sleep efficiency than did European American ethnic status children. Bivariate relations (controlling for sex, age, and ethnicity) between sleep variables and child adaptation scores showed that sleep duration was positively associated with peer acceptance, social skills, social engagement, receptive vocabulary, and understanding of the causes of emotions. Fewer variables were associated with nighttime sleep quality and variability and these tended to be related to outcome variables suggestive of behavioral and emotional regulation. Results suggest that sleep parameters are broadly implicated in the adjustment of preschool age children.