PHILADELPHIA — In the largest sample of healthy adults studied to date under controlled laboratory conditions, researchers at the Perelman School of Medicine at the University of Pennsylvania have found that adults who routinely had late bedtimes and chronic sleep restriction may be more susceptible to weight gain due to the increased consumption of calories during late-night hours. The study results are published in the July issue of the journal SLEEP.

"A number of epidemiological studies have found an association between short sleep duration and weight gain, and ultimately obesity,” said senior study author Namni Goel, PhD, research associate professor of Psychology in Psychiatry, Division of Sleep and Chronobiology at Penn. “We wanted to examine this in a controlled experimental study to determine whether we would observe weight gain over a short period of time when subjects were sleep-restricted. We also sought to determine the source of such weight gain--that is, whether it was due to an additional intake of calories beyond what was needed to maintain body weight."

The study was conducted in the Sleep and Chronobiology Laboratory at the Hospital of the University of Pennsylvania and included 225 healthy, non-obese individuals, ranging in age from 22-50 years. Subjects were randomized to either a sleep restriction or control condition and spent up to 18 consecutive days in the laboratory. Sleep-restricted subjects who spent only four hours in bed from 4 a.m. to 8 a.m. for five consecutive nights gained more weight than control subjects who were in bed for 10 hours each night from 10 p.m. to 8 a.m. The study found an overall increase in caloric intake during sleep restriction, which was due to an increase in the number of meals consumed during the late-night period of additional wakefulness. Furthermore, the proportion of calories consumed from fat was higher during late-night hours than at other times of day.

“In our study, we found that when adults restrict their sleep by delaying their bedtime and staying up late, they are at increased risk for weight gain because they consume a substantial amount of food and drink late at night which is higher in fat than food and drink consumed during morning, afternoon or evening,” said lead study author Andrea Spaeth, MA, a graduate student working in the Sleep and Chronobiology Laboratory. “This late-night eating contributes to weight gain by not only increasing overall daily intake but also by disrupting the timing of caloric intake.”

Additional Penn co-authors include David F. Dinges, PhD.

For more information, please see the American Academy of Sleep Medicine news release.