Lack of sleep and plenty of readily available food can lead to unexpected weight gain.
A new study conducted at the University of Colorado, Boulder showed that people who slept up to five hours a night during a workweek and had unlimited access to food gained almost two pounds.
"Just getting less sleep, by itself, is not going to lead to weight gain," study lead Kenneth Wright, director of CU-Boulder's Sleep and Chronobiology Laboratory, said in a press release. "But when people get insufficient sleep, it leads them to eat more than they actually need."
The National Sleep Foundation says there's "no magic number" for how much sleep you need, but two studies have suggested that adults need about seven to eight hours of sleep a night.
Researchers studied 16 young, lean, healthy adults. They were asked to live two weeks at the University of Colorado Hospital, where they were monitored in a "sleep suite" which had regulated lights and a quiet environment. It also measured the amount of energy the participants were expending based on the amount of oxygen and carbon dioxide they breath in and out respectively.
For the first three days, participants were allowed to sleep up to nine hours a night and have meals that had the same amount of caloric intake required to maintain their weight.
Then the participants were split into two groups, one that was allowed to sleep up to nine hours and another that was only allowed to sleep five hours a night. They were both offered larger meals, had different snack options ranging from fruit and yogurt to ice cream and potato chips. After the five days, the groups switched.
Participants who slept up to five hours a night used 5 percent more energy and ate six fewer calories than those who slept up to nine hours a night. People with less sleep had smaller meals at breakfast, but ate more after-dinner snacks. The late night treats made up more calories for the shortened-sleep group than any other individual meal. Both men and women gained weight when they were only allowed up to five hours of sleep a night.
"When people are sleep-restricted, our findings show they eat during their biological nighttime when internal physiology is not designed to be taking in food," Wright said.
When men had good amounts of sleep and unrestricted food, they gained weight. However, women who had the adequate sleep were more likely to maintain their weight no matter how much grub was placed in front of them.
Proceedings of the National Academy of Sciences on March 11.