Researchers found that when they put 14 volunteers on a schedule that simulated night-shift work, it quickly curbed the number of calories their bodies burned every day.
On average, they expended 52 to 59 fewer calories on "night shift" days, the researchers reported in the current online edition of the Proceedings of the National Academy of Sciences.
That's a fairly small difference -- but one that could add up over time, according to senior researcher Kenneth Wright, of the University of Colorado at Boulder.
The findings, he said, offer one explanation for the negative health effects linked to shift work. Past studies have shown that people on night or rotating shifts have heightened rates of obesity, diabetes and other health problems.
"Shift work goes against our fundamental biology," Wright said. And blunted calorie-burning, he added, may be one of the consequences.
A dietitian who was not involved in the study agreed. "We've recognized for years that when people go on the night shift, they gain weight," said Lauri Wright, an assistant professor at the University of South Florida in Tampa.
"We've tended to look at possible behavioral reasons," said Wright, who is also a spokesperson for the Academy of Nutrition and Dietetics. "Do they eat more to stay awake? Are they too tired to exercise when they're off of work?"
She stressed that food choices, lack of exercise and other lifestyle factors are important. "But," she said, "this study clearly shows there are physiological effects" of a night-shift schedule.
For the study, Wright and his colleagues recruited 14 people to live at their sleep lab for six days. First, the volunteers spent a couple of days on their normal schedule. Then they switched to a night-shift schedule, staying awake at night and sleeping during the day.
The participants' meals were carefully controlled, so they took in the same number of calories each day. Despite that, the researchers found, people's calorie-burning declined on the days they followed a night-shift schedule.
What is going on? It's not completely clear, according to Wright. But he said the lower calorie-burning is probably related to the fact that shift work goes against the body's natural circadian rhythms -- the internal "clocks" that govern fundamental physiological functions, including metabolism.
Still, Wright acknowledged that since his volunteers were kept in a controlled setting for just a few days, it's not clear if the same metabolic changes affect people who really do shift work.
"Are these acute changes?" he said. "Does the body adapt over time? How does it change? This research is really in its infancy, and we have a lot to learn."
However, Wright also noted that shift workers do switch to normal daytime routines on their days off. So their biological clocks wouldn't have the chance to flip to a new norm.
And where does this leave shift workers? Wright, the dietitian, said that people can do their best to eat healthy foods and fit time in for exercise when they're not at work.
"It's very important to include exercise, because it can help offset those changes in the body's calorie expenditure," she said. That could mean taking a walk with your family, she noted, since people who work night shifts usually want family time on their days off.
She also suggested shift workers eat smaller amounts of food, spread out over their waking hours, to keep up their energy levels without overindulging. "And make sure you get plenty of water and fiber -- from foods like fruits, vegetables, whole grains and beans," Wright said. "Both are good for appetite suppression."
Employers could help, she added, by educating shift workers on healthy habits and making it easier to choose nutritious foods. "When I worked in the hospital setting," she said, "the cafeteria would be closed at night, and I'd see people relying on vending machines."
For his part, Wright stressed the importance of adequate sleep. "We usually recommend getting five to six hours of sleep right after your shift ends," he said. "Then try to nap for about two hours later in the day, before your next shift."
He agreed that wise food choices and regular exercise are especially important for shift workers -- though it can be difficult. "It's a huge challenge," Wright said. "Unfortunately, there's no easy answer."
The National Sleep Foundation has more on shift work.