During adolescence, there is a tendency to sleep late and sleep less because of altered psychosocial and life-style changes. Recent studies have demonstrated the link between sleeping less and gaining weight in children, adolescents, and adults. We studied the effect of late sleeping and sleeping less on body mass index (BMI) in medical college freshmen. All participants were adolescents (104 male and 38 female adolescents, mean age 17.77±0.79 years). After obtaining informed consent, they filled out a questionnaire about their sleeping habits. Height and weight were measured after a brief history taking and clinical examination. BMI increased significantly with decrease in total sleep duration and with delayed bedtime. Late sleeping individuals (after midnight) had significantly less sleep duration (6.78 hours v 7.74 hours, P<0.001), more day time sleepiness (85.2% v 69.3%, P=0.033) and more gap between dinner time and going to sleep (234.16 min v 155.45 min, P<0.001). Increased BMI in late sleepers may be explained by low physical activity during the day caused by excess sleepiness and increased calorie intake with a gap of 5-6 hours between dinner and sleep. Sleep habits of late sleeping and sleeping less contribute to increase BMI in adolescents.