Background There were increasing evidence supporting the presence of the relationship between sleep duration and obesity. However, whether a negative linear trend or a U-shaped pattern could explain the relationship has been a topic of debate.

Objectives To examine whether the possible association between sleep duration and obesity is U-shaped among school-aged children.

Participants and methods A random sample of 20,778 children aged 5.01 to 11.99 years participated in a cross-sectional survey conducted in eight cities of China. The Chinese version of the Children’s Sleep Habits Questionnaire was used to collect information on children’s sleep behaviors.

Height and weight were measured and body mass index (BMI) was calculated. Overweight/obesity was defined by the standardized internationally referenced gender- and age-specific BMI cut-offs.

Results The prevalence of overweight and obesity in our sampled school-aged children was 11.7% and 7.1%, respectively. There was a significant U-shaped relationship between sleep duration and overweight/obesity after adjusting for age, gender, parents’ educational levels, family income, media-use, homework schedule, and physical activity. The estimated nadir of the sleep duration curve was approximately 9.4 hours/d for boys and approximately 9.6 hours/d for girls. Interestingly, the U-shaped relationship showed different characteristics between boys and girls. Moreover, dose-effect trend was observed both in boys and girls.

Conclusions Both short and long sleep duration maybe independently associated with a higher risk of overweight/obesity in children, indicating sleep plays a precise and complicated, although unclear, role in the regulation of energy metabolism.