The prevalence of overweight and obesity in children has risen greatly worldwide. Diet and poor physical activity are the two risk factors usually examined, but epidemiological evidence exists suggesting a link between sleep duration and overweight/obesity in children. The aim of this study was to describe the relationship among body mass index (BMI), diet quality, physical activity level, and sleep duration in 690 children attending the 5th grade in primary schools (9-11 years old) in the city of Parma (Italy) involved in the Giocampus educational program. This was achieved through (i) measuring anthropometric data to compute body mass index; (ii) administering a food questionnaire to evaluate adherence to the Mediterranean Diet (KIDMED score); and (iii) administering a lifestyle questionnaire to classify children physical activity level (PAL), sleep duration, and school achievement. A highly significant negative association was found between BMI and sleep hours. Moreover, there was a significant positive association between PAL and KIDMED scores. No evidence was found of association between BMI and PAL, nor between BMI and KIDMED score. Data from this study established that BMI is correlated to gender and sleep duration, defining sleep habits as one of the factors linked to overweight and obesity.