Background: Many reports argue that sleep is important for children's health, learning, and academic performance. The purpose of this longitudinal study was to examine the association between sleep and the development of social competence in infants. Methods: This study was conducted as part of a Japan Science and Technology Agency (JST) project. Caregivers responded to the Japan Children's Study Sleep Questionnaire when children were 18 months old. The interactions of caregivers and children were observed when children were 18, 30, and 42 months old, and rated with the Interaction Rating Scale, which is a measure of social competence. Results: Nocturnal sleep duration of more than 10 h and an earlier bed time than 22:00 were significantly correlated with two trajectory groups (low point and high point transition groups) of children's social competence at 18, 30, and 42 months. Further, total sleep duration of more than 12.25 h and an earlier bed time than 22:00 were significantly correlated with the trajectory of children's social competence at 18, 30, and 42 months. Conclusions: Sleep duration and sleep onset time are important factors in children's development of social competence. Trial registration The ethics committee of the JST approved this study on March 19, 2001. The registration number is 356-1.
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