BACKGROUND: Sleep duration is attracting increasing attention in relation to chronic disease risk, but few large-scale studies have investigated the determinants of sleep characteristics in early life. In this study we used data from a large, population-based twin study to examine genetic and environmental influences on sleep duration and sleep difficulties in infancy. METHODS: Participants were 1931 pairs of young twins (3862 children) from the Gemini twin birth cohort. Sleep patterns were assessed at 15 months by using a modification of the Brief Infant Sleep Questionnaire completed by parents. Outcomes included nighttime and daytime sleep duration and frequency of night waking. RESULTS: Twin analyses showed that nighttime sleep duration was predominantly influenced by the shared environment (66%, confidence interval [CI] 63%-70%) with a modest genetic effect (26%, CI 22%-30%). A similar pattern was observed for daytime nap duration (shared environment: 57%, CI 53%-62%; genetic effect: 37%, CI 33%-41%) and sleep disturbance (shared environment: 55%, 44%-64%) with a genetic effect of 40% (30%-51%). These estimates were similar for boys and girls. CONCLUSIONS: These results indicate an important contribution of the shared family environment as well as genes to children's sleep behavior. There is a need for research to identify specific environmental determinants that could provide targets for interventions to improve sleep quality.
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