A new study published in the journal Sleep has linked late nights and less sleep with a susceptibility for weight gain.

The study, conducted by researchers at the Sleep and Chronobiology Laboratory at the University of Pennsylvania, showed that patients whose sleep was restricted gained more weight than those who slept long stretches. Patients who were restricted to sleeping only four hours, from 4 am to 8 am, gained more weight than patients who were allowed to sleep 10 hours a night, from 10 pm to 8 am. The study’s authors attributed this weight gain to the extra calories eaten late at night by sleep-restricted patients. In addition the calories from fat consumed by sleep-restricted was higher late at night.

“Although previous epidemiological studies have suggested an association between short sleep duration and weight gain/obesity, we were surprised to observe significant weight gain during an in-laboratory study,” said Andrea Spaeth, lead author of the study and a doctoral candidate in the psychology department at the University of Pennsylvania.

The study put 225 healthy, non-obese patients in a sleep laboratory, randomly assigning them to have four or ten hours of sleep for 18 consecutive days. Meals were served at set times, but food was always available in the lab’s kitchen. The patients were allowed to do sedentary activities such as reading, watching TV, or playing video games, but were not allowed to exercise.

In addition to the weight gain link, certain gender and ethnic differences were uncovered during the study. Men were found to gain more weight than females when sleep-restricted, and sleep-restricted black patients also gained more weight than sleep-restricted white patients.

“Among sleep-restricted subjects, there were also significant gender and race differences in weight gain,” said Spaeth. “African Americans, who are at greater risk for obesity and more likely to be habitual short sleepers, may be more susceptible to weight gain in response to sleep restriction. Future studies should focus on identifying the behavioral and physiological mechanisms underlying this increased vulnerability.”