Preschool children's sleep was examined as a moderator of the association between negative emotionality and both peer acceptance and peer rejection. Participants were 115 children (47 percent girls, M age = 4.29 years, SD = .63). Preschool teachers reported on children's negative emotionality (anger/frustration, sadness, and fear). Sleep was measured objectively using actigraphy in the child's home for seven consecutive nights. Peer acceptance and rejection were assessed using children's choices in sociometric interviews. Controlling for potential confounds, moderation analyses revealed that negative emotionality predicted peer acceptance and rejection only among children with poorer sleep quality (lower sleep efficiency, more frequent wake episodes, longer sleep latency), but not better sleep quality. Findings suggest that sleep is important not only for predicting child functioning but also for moderating the adverse effects of negative emotionality on a salient indicator of interpersonal functioning for preschool age children.