Andrea Spaeth, a graduate student working in the Sleep and Chronobiology Laboratory of the University of Pennsylvania's Perelman School of Medicine in Philadelphia, said the study involved 225 people ages 22-50, who spent 18 days in a sleep lab.
Meals were served at scheduled times, and food was always available in the laboratory kitchen for participants who wanted to eat at other times of day. Subjects could move around but were not allowed to exercise. They were permitted to watch TV, read, play video games or perform other sedentary activities.
Sleep-restricted subjects who spent only 4 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, the study found.
"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," Spaeth said in a statement.
"This late-night eating contributes to weight gain by not only increasing overall daily intake but also by disrupting the timing of caloric intake."
When the study subjects were up from 10 p.m. to 4 a.m., they consumed an additional 550 calories, compared with those who sleep regular hours, Spaeth said.
The average weight gain for the sleep-restricted people was a couple of pounds, Spaeth said.
The study was published in the journal Sleep.