Temperamental negative affect and insufficient sleep have been independently associated with behavior problems during early childhood. However, it is unknown whether these factors interact to contribute to behavioral difficulties in young children. The current study examined the interactions between temperamental negative affect and both sleep onset time and sleep midpoint, assessed by actigraphy, in predicting externalizing and internalizing behaviors in a sample of 117 children (34–69 months of age). Children with high temperamental negative affect and either later sleep onset time or later sleep midpoint were more likely to exhibit externalizing and internalizing behaviors. These results emphasize the association between temperamental negative affect and behavioral difficulties, particularly for children with insufficient sleep.