The current study aimed to examine infant sleep duration as a moderator of the relations between maternal sensitivity and child externalizing and internalizing symptoms, in a prospective longitudinal design. Fifty-five Caucasian infants (33 girls) took part in 2 assessments, at 1 and 4 years. Maternal sensitivity was rated at 1 year, based on observations performed throughout a home visit. Infant sleep duration (i.e., nighttime sleep duration and 24-hr sleep duration) was assessed at 1 year as well, using a sleep diary completed by mothers. At 4 years, mothers completed the Child Behavior Checklist to assess children's internalizing and externalizing symptoms. Results indicated that maternal sensitivity interacted with infant nighttime sleep duration, such that there were negative relations between sensitivity and subsequent internalizing and externalizing symptoms only for children who slept more at night. Interactions using 24-hr sleep duration as the moderator were not significant. These findings add to an emerging literature on the importance of sleep for children's daytime functioning by suggesting that inadequate or insufficient sleep in infancy can interfere with the normal developmental process linking early maternal sensitivity to child subsequent emotional and behavioral adjustment. (Contains 3 tables and 1 figure.)