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To determine whether reduced sleep is associated with differences in body composition and the risk of becoming overweight in young children.Longitudinal study with repeated annual measurements.Dunedin, New Zealand.244 children recruited from a birth cohort and followed from age 3 to 7.Body mass index (BMI), fat mass (kg), and fat free mass (kg) measured with bioelectrical impedance; dual energy x ray absorptiometry; physical activity and sleep duration measured with accelerometry; dietary intake (fruit and vegetables, non-core foods), television viewing, and family factors (maternal BMI and education, birth weight, smoking during pregnancy) measured with questionnaire.After adjustment for multiple confounders, each additional hour of sleep at ages 3–5 was associated with a reduction in BMI of 0.48 (95% confidence interval 0.01 to 0.96) and a reduced risk of being overweight (BMI ≥85th centile) of 0.39 (0.24 to 0.63) at age 7. Further adjustment for BMI at age 3 strengthened these relations. These differences in BMI were explained by differences in fat mass index (−0.43, −0.82 to −0.03) more than by differences in fat free mass index (−0.21, −0.41 to −0.00).Young children who do not get enough sleep are at increased risk of becoming overweight, even after adjustment for initial weight status and multiple confounding factors. This weight gain is a result of increased fat deposition in both sexes rather than additional accumulation of fat free mass.