Teenagers who go to bed late are more likely to pile on the kilograms, according to new research. A five-year study found that every extra hour they stayed up during the school week added an alarming 2.1 more points to their BMI (body mass index).And exercise, the amount of time they spent in front of computer or TV screens and the actual number of hours they slept made no difference.The study of more than 3300 participants said teenage night owls might want to hit the hay earlier in a bid to combat obesity.Lauren Asarnow, of the University of California, Berkeley, said: "These results highlight adolescent bedtimes, not just total sleep time, as a potential target for weight management during the transition to adulthood."BMI is the measure of a person's weight in kilograms divided by the square of height in metres. A healthy adult BMI range is estimated to be 18.5 to 24.9. Above this is considered overweight, and above 30 obese.The study, published in the journal Sleep, analysed data from the National Longitudinal Study of Adolescent Health, which has tracked the influences and behaviours of US teen-agers.It focused on the onset of puberty, the high school years and young adulthood and compared the bedtimes and BMI of the participants from 1994 to 2009.Adolescents in the study reported their bedtimes and sleep hours, while researchers calculated their BMI based on their height and weight.Surveys show that many teenagers do not get the recommended nine hours of sleep a night, and report having trouble staying awake at school.The human circadian rhythm, or body clock, which regulates physiological and metabolic functions typically shifts to a later sleep cycle at the start of puberty.Asarnow added that the results suggest adolescents who go to bed earlier will "set their weight on a healthier course as they emerge into adulthood".She is a researcher on the university's Teen Sleep Study, a programme designed to reset the biological clocks of adolescents who have trouble going to sleep and waking up. ..