A random sample of 291 9- and 10-year-old schoolchildren from Asturias (Spain) was taken. Using path analysis, a model was tested in which bedtime, the number of hours spent sleeping and leisure activities were the independent variables and the body mass index was the dependent variable. The results show that sedentary and active leisure time and hours spent sleeping are predictors of the body mass index in children. Those children who go to bed late and who use that extra time to watch the television or play with the computer tend to have a greater body mass index, while those children who go to bed earlier and have spent more time reading or playing in the park or at home have a lower body mass index. Encouraging active leisure activities can have an extremely positive effect on their body mass index.