Although a number of studies have investigated the relationship of sleep duration to overweight and obesity, studies conducted among population-based elementary schoolchildren have been limited in Japan. The aim of the present study was to investigate the relationship between sleep duration and overweight among elementary schoolchildren in Japan. The study subjects were all fourth-grade schoolchildren (9 or 10 years of age) in Ina-town, Saitama Prefecture, Japan from 1999 to 2008. Information concerning each subject's sex, age, and lifestyle was obtained using a self-administered questionnaire, while measurements of his or her height and weight were carried out. Childhood overweight was determined according to the definition established by the International Obesity Task Force. Data from 3,433 children were analyzed. In logistic regression analysis, a statistically significant dose-response relationship was observed between sleep duration and overweight among boys (p for trend = 0.014) but not among girls (p for trend = 0.149). Short sleep duration was associated with childhood overweight, and the sex difference in the association was observed. These findings suggested that it is important to consider sleep duration as part of any program to prevent overweight among elementary schoolchildren, especially among boys.