U.S. teens are more sleep deprived than ever. Doctors say adolescents need 9 to 9½ hours of sleep each night, but most are getting much less than that -- closer to seven hours. Studies have shown that the consequences can be devastating in this critical period of growth and include everything from lower academic performance to a higher risk of emotional distress.

There's been movement in many school districts across the nation to get schools to start later to increase the amount of time teens can stay in bed, but the results of a new study suggest that this may not completely solve the sleep problem.

The study, published in this month's issue of the journal Sleep, found that bedtimes rather than duration of sleep may be linked to at least one health consequence: weight gain.

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The researchers followed 3,342 adolescents from 1996 to 2009 and interviewed them three times during the study period about their bedtimes, food consumption, exercise and television watching. They also measured the volunteers' heights and took their weights to calculate their body mass index (BMI) during each of the check-ins.

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They found that the later the average bedtime during the schoolweek, the higher the BMI over time. More precisely, they found that for every minute of later bedtime there was an increase in BMI of 0.035 kg/m2. Or for every additional hour later a 2.1 increase in BMI.

The results were significant even after they controlled for things like gender, race and socioeconomic status. Interestingly, sleep duration, screen time and exercise frequency didn't seem to affect BMI over time.

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"The results are important because they highlight adolescent bedtimes, not just total sleep time, as a potential target for weight management concurrently and in the transition to adulthood," Lauren Asarnow, a doctoral candidate at the University of California at Berkeley and co-author of the study, said in a statement.

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