By Sudeep Chand
Health reporter, BBC News
Eating in the middle of the night has been linked to obesity
Late-night snackers are more likely to gain weight, research suggests.
A team from Northwestern University, Illinois, found that when you eat, not just how you eat, could make a big difference.
Scientists found that when mice ate at unusual hours, they put on twice as much weight, despite exercising and eating as much as others.
The study, in the journal Obesity, is said to be the first to show directly that there is a "wrong" time to eat.
Recent studies have suggested that circadian rhythms, the body's internal clock, have a role in how our bodies use up energy. However, this had been difficult to definitively pin down.
Deanna Arble, lead author of the study, said: "One of our research interests is shift workers, who tend to be overweight.
"This got us thinking that eating at the wrong time of day might be contributing to weight gain."
The experiment looked at two groups of mice over a six-week period. Both groups were fed a high-fat diet, but at different times of the mice "waking cycle".
One group of mice ate at times when they would normally be asleep. They put on twice as much weight.
This was despite them doing the same level of activity, and eating the same amount of food, as the other mice.
The findings may have implications for people worried about their weight.
"How or why a person gains weight is very complicated, but it is clearly not just calories in and calories out," said Fred Turek, from the Northwestern's Center for Sleep and Circadian Biology, where the research took place.
"Better timing of meals could be a critical element in slowing the ever-increasing incidence of obesity."
Tam Fry, from the National Obesity Forum, agreed. He said: "It is groundbreaking. It really gets you thinking why this has not been done before.
"It could be very dramatic if it affects whether you are going to get fat or not."
At this stage, the results could still be interpreted as controversial when applied to humans.
The scientists now hope they can find out more about how the process works. It is thought that sleep, hormones and body temperature all play a part in how we gain weight.