Why working nightshifts is an obesity risk
Scientists believe they have discovered why people who work nightshifts are more prone to weight gain and obesity.
Rather than eating more, night workers burn off fewer calories, their research suggests.
The scientists told the journal PNAS eating at night messed with our body's natural metabolism.
The researchers set up a small trial where volunteers ate identical amounts as they switched between working days and nights.
For the first two days, the volunteers followed a normal schedule - sleeping at night, staying awake during the day, and eating breakfast, lunch and an evening meal.
For the next three, they stayed awake and ate their meals at night and slept during the day.
Although the timing of the meals changed, the calorie content remained the same.
Total energy expenditure - how much energy (calories) the volunteers burned - went down when they switched to the nightshift pattern.
Even though participants initially burned more fat - possibly because they slept less during the transition to nights, say the researchers from the University of Colorado - the energy expenditure over the three days of shift work was lower overall.
Lead investigator Prof Kenneth Wright says this is because shift work goes against our fundamental biology.
Our internal body clock makes us better suited to eating during daylight hours.
"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."