Night shift workers may expend less energy and be more prone to weight gain than people who have normal work schedules, reveals a new study published in the journal Proceedings of the National Academy of Sciences.

Researchers at the University of Colorado at Boulder studied 14 healthy adults who spent six days at the university hospital’s Clinical and Translational Research Center. For the first two days of their stay, they slept at night and stayed awake during the day. After that, they reversed their routines and transitioned to shift work. They slept eight hours every day and consumed the same number of daily calories throughout the study as they did prior to the study. But after beginning shift work, researchers found the participants burned less energy than they did during their normal schedules.

"When people are on a shift work-type schedule, their daily energy expenditure is reduced, and unless they were to reduce their food intake, this by itself could lead to weight gain," senior study author Kenneth Wright, director of CU-Boulder's Sleep and Chronobiology Laboratory, said in a news release.

Wright hypothesized that  the reduction in energy expenditure is linked to a mismatch between the person’s activities and their circadian clocks. The circadian clock is largely set by exposure to light, and humans have evolved to be awake and eat when there’s light outside and to sleep when it’s dark.

"Shift work goes against our fundamental biology," said Wright, who is also an associate professor of integrative physiology. "Shift work requires our biological day to occur at night and our biological night to occur during the day and that's very difficult to achieve because the sun is such a powerful cue. We can have some change in our clock— a couple of hours— but then on days off, it goes right back. Shift workers never adapt."

Researchers said the most surprising finding was that study participants burned more fat when they slept during the day compared to when they slept at night. Wright said it may be possible the extra fat burning is triggered by the transition day between a daytime and nighttime schedule— a time when shift workers often take an afternoon nap to prepare for their night shift. However, in doing so, they are awake for more hours than usual and burn more energy. The extra demand for energy subsequently causes the body to begin burning fat, Wright said.

Despite this fat-burning phenomenon, researchers said that the participants’ energy expenditure over the three days of shift work was lower than when they had normal schedules.

Wright said further research needs to be done on actual shift workers whose diets aren’t controlled, but he said the findings are a good starting point for making recommendations to improve shift workers’ health.

"What we can say is that it's perhaps even more important to have a healthy diet for shift workers as well as a healthy amount of physical activity," he said.