Now a new US study says people who work the night shift, and therefore eat at night, are likely to burn less energy during a 24-hour period than those on a normal schedule, increasing their risk of putting on weight.
For the new study 14 healthy adults (8 women and 6 men) spent 6 days at the University of Colorado Hospital's Clinical and Translational Research Center. For the first 2 days, they followed a normal schedule, sleeping at night and staying awake during the day. They then transitioned to a 3 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 number of calories remained the same.
Putting on the pounds
Senior study author, Kenneth Wright, says in a press release: "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."
He says the reduction in energy expenditure is probably linked to the mismatch between a shift worker's activities and their body (circadian) clocks. Humans have evolved to be awake, and eat, when it's light outside and sleep when it's dark.
"Shift work goes against our fundamental biology," says Kenneth Wright. "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 research team from University of Colorado Boulder was surprised to find that the people in the study burned more fat when they slept during the day compared to when they slept at night. It's not clear why this happens, but it's possible the extra fat-burning is triggered by the transition day between a daytime schedule and a night-time schedule.
Kenneth Wright cautions that even though participants initially burned more fat, this would not lead to weight loss because in total, the energy expenditure over the 3 days of shift work was lower.
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.
Kenneth Wright says more work is needed before specific recommendations can be made for how to improve the health of shift workers but the new study provides a starting point.
"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."
The findings have been published in the Proceedings of the National Academy of Sciences.
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