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Night shifts have previously been linked to negatively impacting health and now, another study is adding more evidence supporting that link. According to a team headed by researchers from the University of Colorado Boulder, people who work the night shift burn less energy and are at a greater risk of weight gain when compared to people who work normal daytime hours.

For this study, the researchers recruited 14 healthy adults for a six-day experiment that took place at the University of Colorado Hospital's Clinical and Translational Research Center. For the first two days, the participants slept normally. Their sleeping schedule was then changed to a three-day shift work, which forced them to work at night and sleep during the day. However, they still had the opportunity to sleep for eight hours.

Throughout the experiment, the researchers controlled meals so that the participants ate the same amount of food that they would normally consume in order to maintain weight. Regardless of when the participants ate, overall calorie consumption was stable.

The researchers found that once participants changed to a shift work schedule, the total daily amount of energy that they used up declined. According to senior author of the paper, Kenneth Wright reported in the press release, the decline could be due to the "mismatch between the person's activities and their circadian clocks."

"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," said Wright, director of CU-Boulder's Sleep and Chronobiology Laboratory. "Shift work goes against our fundamental biology. 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."

The team did find that people who slept during the day ended up burning more fat. However, this did not lead to weight loss because the participants' overall energy expenditure totaled after three days was considered very still low.

The study was published in the Proceedings of the National Academy of Sciences.