Researchers say there's another reason to hit the hay early: going to bed later during the workweek is associated with weight gain over time, according to a study published in the October issue of the journal Sleep.

A team of researchers analyzed data on more than 3,300 teens and young adults recorded at different intervals over the course of about 15 years.

"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," study author Lauren Asarnow, a doctoral candidate at the University of California, Berkeley, told CBS News. "There's been some literature looking at the relationship late bedtimes and weight gain cross-sectionally, but no one's ever looked at what happens long term."

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The results showed that every hour of sleep lost was associated with a 2.1 point increase in body mass index (BMI). This gain occurred roughly over a five-year period.

According to the National Institutes of Health, a healthy adult BMI range is estimated to be between 18.5 and 24.9. A BMI of 25 to 29.9 is considered overweight, while a BMI of 30 or greater constitutes obesity.

"Conceivably, if you're going to bed an hour later, over time you could be shifting BMI categories from normal to overweight," Asarnow said. "So even a two-point increase could be clinically significant."

What's more, the study showed a surprising result: the relationship between bedtime and BMI was not significantly changed or moderated by total sleep time, exercise frequency or screen time. Sleeping late did not make up for the impact of a late bedtime.

This is important, Asarnow said, because it highlights "adolescent bedtimes, not just total sleep time, as a potential target for weight management concurrently and in the transition to adulthood."

A couple theories exist for why later bedtimes affect weight gain. The first is behavioral related. "If you're staying up late you're more likely to be eating junk food late at night," Asarnow said. "People who stay up late are also less likely to eat breakfast and breakfast skipping is associated with weight gain."

Additionally, previous studies have shown that a chronic pattern of late sleep times contributes to metabolic disturbances.

Asarnow said the current study has important implications. "The good news is sleep is a highly modifiable," she said. "If you could shift bedtime in the teenage years, you can create good sleep habits and maybe prevent weight gain over time."