Have you been burning the midnight oil lately? In a culture that values work and productivity, getting to sleep late on weeknights is an inevitable part of American lives. According to a new study from researchers at the University of California Berkeley, however, this may mean bad news if you’re trying to keep your weight down.

The study shows that there was a distinct correlation between the amount of sleep people get, when they go to sleep, and body mass index, a measure comparing weight relative to a person’s height. Researchers in the study examined data from a sample of over 3,300 young adults and adults to determine just how much of an influence bedtime had in keeping off excess weight.

The study found that for each hour of sleep a person lost on the front end, they were likely to gain an average of 2.1 points on their body mass index. The study followed the sample over a five-year timeframe and tracked their sleeping habits and overall change in BMI.

The study revealed that the amount of exercise, the amount of time spent in front of a screen, and the overall number of hours slept didn’t have nearly as large of an effect on BMI increase as the time of night people went to sleep.

According to the study’s lead author and doctoral student at UC Berkeley’s Golden Bear Sleep and Mood Research Clinic, Lauren Asarnow, adolescents faced the biggest challenge in keeping off excess weight while maintaining a late bedtime. The body mass index is calculated by dividing the weight of a person in kilograms by the square of their height in meters. Adults considered to be healthy have a BMI that ranges from 18.5 to 24.9.

The study pulled data from the National Longitudinal Study of Adolescent Health, a program that has been recording data about the health and habits of American teens since 1994. The Berkeley researchers compared the bedtimes and the body mass indices of teenagers from 1994 to 2009 at three key points in their transition to adulthood – the beginning of puberty, the college age group, and young adulthood. Teens and young adults were asked to record their bedtimes and total hours spent sleeping, which were compared to years of data tracking the changes in body mass index.