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People who work night shifts are more likely to pile on the pounds because they burn less energy, scientist claim.

Past studies have shown that those who work, and therefore eat, at night when their bodies are biologically prepared to sleep are prone to weight gain, the Daily Mail reports.

But research has failed to provide a clear reason.

Now, U.S. researchers say they have a reason - and it's because shift workers overall burn less energy - and unless they reduce their food intake, it is enough to cause weight gain.

Researchers at the University of Colorado Hospital examined 15 healthy adults over six days.

For the first two days, the group followed a normal schedule, sleeping at night and staying awake during the day.

They then switched to a three-day shift work schedule, when their routines were reversed.

Senior author Dr Kenneth Wright said: '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.'

The 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 they switched to the shift work schedule, the timing of their meals changed but the total amount of calories remained the same.

Those taking part were also given the same eight-hour sleep opportunity, regardless of whether those hours were scheduled during the day or night.

The scientists found that total daily energy used by the participants decreased when they started evening shift work.

Dr Wright said the reduction is probably linked to the mismatch between the person's activities and their body clocks.

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.

However, 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.

Dr Wright said: '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 research team was surprised to find that the study participants burned more fat when they slept during the day compared to when they slept at night.

It's not clear why this happens, but Dr Wright said it's possible the extra fat-burning is triggered by the transition day between a daytime and a night-time schedule.

He added that on that day shift workers often take an afternoon nap to prepare for the first nightshift, but in total, they are typically awake more hours than usual and, therefore, burn more energy.

The need to meet the extra demand for energy may cause the body to begin burning fat.

However, Dr Wright cautioned that even though participants initially burned more fat, this would not lead to weight loss because in total, the energy expenditure over the three days of shift work was lower.

He added: '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 new study was published in Proceedings of the National Academy of Sciences.