Specifically, the findings show that teens who go to bed late on weeknights are more likely to gain weight compared to those who go to sleep earlier. This late-bedtime link remained consistent regardless of how many hours the teens actually slept.
Researchers from the University of California, Berkeley, analyzed longitudinal data from a national sample group of more than 3,300 youths and adults, and found that for every extra hour they stayed awake, they gained 2.1 points on the BMI index. This gain occurred roughly over a five-year period.
Furthermore, exercise, screen time, and the number of hours they slept did not affect this BMI increase.
“These results highlight adolescent bedtimes, not just total sleep time, as a potential target for weight management during the transition to adulthood,” said Lauren Asarnow, lead author of the study and a doctoral student in University of California, Berkeley’s Golden Bear Sleep and Mood Research Clinic.
BMI is the measure of a person’s weight in kilograms divided by the square of height in meters. A healthy adult BMI range is estimated to be 18.5 to 24.9.
The Berkeley study analyzed data from the National Longitudinal Study of Adolescent Health, which has tracked the influences and behaviors of U.S. teenagers since 1994. Focusing on three time periods - the onset of puberty, the college-age years and young adulthood - researchers compared the bedtimes and BMI of teens from 1994 to 2009.
Participants in the study reported their bedtimes and sleep hours while researchers calculated their BMI based on their height and weight.
According to the National Sleep Foundation, teens tend to have irregular sleep patterns across the week. They typically stay up late and sleep in late on the weekends, which can affect their biological clocks and hurt the quality of their sleep.
This is confirmed by surveys in which teens report that they do not get the recommended nine hours of sleep a night, and report having trouble staying awake at school. The human circadian rhythm, which regulates physiological and metabolic functions, typically shifts to a later sleep cycle at the onset of puberty.
The findings of the study thus suggest that adolescents who go to bed earlier will “set their weight on a healthier course as they emerge into adulthood,” Asarnow said.
The research is published in the journal Sleep.
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