Sleeping Time, BMI, and Body Fat in Chinese Freshmen and Their Interrelation

Introduction: In the past two decades, urbanization in many Asian countries has led to sedentary lifestyle and overnutrition, which has set the stage for the epidemic of obesity. Those who have obesity during adolescence usually have obesity into adulthood, which causes many medical and psychological issues that can result in premature death. Recent data suggest that short sleep duration may contribute to the risk of obesity, opening a new avenue for potential intervention. The aim of this study was to describe the prevalence of sleeping time, body mass index (BMI), and fat mass in Chinese freshmen and to indicate the relationship among them.

Methods: We conducted a cross-sectional study consisting of 1,938 freshmen aged from 18 to 24 years, including 684 (35.3%) men and 1,254 (64.7%) women. BMI, sleeping time, and other potential related variables were collected by questionnaire. Obesity and overweight were defined as BMI ≥28.0 and ≥24.0, respectively. All students were divided into three groups according to the tertiles of body fat percentage (Fat%), which were measured by bioelectrical impedance analysis. Sleeping time was divided into 3 categories based on the duration, <6, 6-8, and >8 h per day.

Results: Participants who slept longer had lower BMI and Fat%. Sex, diet control, and maternal obesity were all significantly associated with BMI or Fat% after adjustment. There were significant differences between sleeping time <6 and >8 h in both overweight group and Fat% tertiles independent of adjustment used. The significant difference between sleeping time 6-8 and >8 h only existed in overweight group, while irrelevant relation was observed in both models of obesity group.

Conclusion: Longer sleeping time was associated with a lower BMI in Chinese freshmen, even after adjustment for multiple confounders. The overweight stage is a critical period of weight management intervention by changing sleeping time. If adolescents have entered the obesity stage, the effect size of lifestyle intervention might be significantly limited.