People who work the night shift are likely burning less energy during 24 hours than those on a normal schedule, increasing their risk for weight gain and obesity, according to a new study led by the University of Colorado Boulder.

For the new study, published in the Proceedings of the National Academy of Sciences, fourteen healthy adults spent six days at the University of Colorado Hospital's Clinical and Translational Research Center. For the first two days, the participants followed a normal schedule of sleeping at night and staying awake during the day. They then transitioned to a three-day shift work schedule when their routines were reversed.

During the experiment, participants' meals were carefully controlled, and they were given the amount of food they would normally need to eat at home to maintain their current weight. When the participants transitioned to the shift work schedule, the timing of their meals changed but the total amount of calories remained the same.

The participants also were given the same eight-hour sleep opportunity regardless of whether those hours were scheduled during the day or night.

The researchers found that the total daily energy used by participants decreased when they were put on a shift work schedule. The reduction is probably linked to the mismatch between the person's activities and their circadian clocks, Wright said. Humans have evolved to be awake—and eat—when it's light outside and sleep when it's dark. In large part, the human circadian clock is set by exposure to sunlight.

People's circadian clocks can shift over time—even radically—with the use of artificial lights if they aren't exposed to the sun. But because shift workers typically switch back to a daytime schedule on their days off, their biological clocks don't flip to fit their night shift schedules.

Further research is needed to determine if the fat-burning phenomenon would happen among actual shift workers, whose diet is not being strictly controlled, Wright said. For example, shift workers may eat more calories on the transition day—an option not available to study participants—which could eliminate the need for the body to start burning fat. Still, the findings suggest that shift workers may be prone not only to gaining weight but also to a changing composition of fat and muscle mass in their bodies.