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During adolescence, there is a tendency to sleep late andsleep less because of altered psychosocial and life-stylechanges. Recent studies have demonstrated the link betweensleeping less and gaining weight in children, adolescents, andadults. We studied the effect of late sleeping and sleepingless on body mass index (BMI) in medical college freshmen.All participants were adolescents (104 male and 38 femaleadolescents, mean age 17.77±0.79 years). After obtaininginformed consent, they filled out a questionnaire about theirsleeping habits. Height and weight were measured after abrief history taking and clinical examination. BMI increasedsignificantly with decrease in total sleep duration and withdelayed 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 tosleep (234.16 min v 155.45 min, P<0.001). Increased BMI inlate sleepers may be explained by low physical activity duringthe day caused by excess sleepiness and increased calorieintake with a gap of 5-6 hours between dinner and sleep.Sleep habits of late sleeping and sleeping less contribute toincrease BMI in adolescents.