Parents now have another reason why they should get their kids to sleep early. Staying up late on weeknights elevates teens' risk of becoming overweight over time.

Late bedtime could be linked to weight gain in teenagers. Researchers of a new study, which was published in the October issue of Sleep, looked at the data of over 3,300 teens and found an associated 2.1 increase in body mass index (BMI) for each hour that the adolescents went to bed late.

The researchers likewise found that the link between staying up late at night and BMI increase was not significantly affected by the amount of exercise, total sleep time and time that the teens spent in front of screens such as televisions and computers.

Study researcher Lauren Asarnow, from the University of California, Berkeley, said that the findings are important because these show that bedtimes and not just the total sleep time of adolescents can be a potential target for weight management during adolescence and during a person's transition into adulthood.

"Obesity is obviously growing among adolescents and adults, and there's also an epidemic of lack of sleep and later bed time preference in teens," Asarnow said. "There's been some literature looking at the relationship [between] late bedtimes and weight gain cross-sectionally, but no one's ever looked at what happens long term."

Several theories exist as to why later bedtimes can influence weight gain. One is behavior-related. Asarnow said that people who stay up late at night are more likely to eat junk food in the evening. They also tend to skip breakfast, which is associated with weight gain. Earlier studies have likewise shown that late sleep times can results in metabolic disturbances.

The American Academy of Sleep Medicine recommends that teens get a little over nine hours of sleep at night, but surveys have found that many teenagers do not get sufficient sleep at night.

The body's circadian rhythm, which regulates metabolic and physiological functions, appears to contribute to late bedtime among teens as it tends to shift to a later sleep cycle at the onset of puberty.

The popularity of tablets, computers and cellphones among teens also appears to have an impact on the amount of sleep that they get.

Research has shown that exposure to screens before going to bed can affect sleep patterns, prompting experts to encourage that teens get a "screen-free" hour before sleeping.

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