It has long been thought that it’s not just what you eat but when you eat that has an effect on gaining weight. A new study reaffirms this.

Researchers fed mice a high-fat diet during the normal time they ate. Those mice gained about 20 percent of their weight over a six-week period. But, when the researchers fed other mice the same diet, but during the time that they would normally be sleeping, those mice put on 48 percent of their weight.

While these results need to be duplicated in a human study, the researchers believe that the results will be the same.

The popular slogan “calories in/calories out” to describe how you lose weight via a calorie deficit may be oversimplifying it a bit.

“How or why a person gains weight is very complicated, but it clearly is not just calories in and calories out,” says the study’s leader Fred Turek, professor of neurobiology and physiology at Northwestern University. “We think some factors are under circadian control. Better timing of meals, which would require a change in behavior, could be a critical element in slowing the ever-increasing incidence of obesity.”

More research is certainly needed. Not just because the findings need to be duplicated in a human study, but a similar study involving monkeys concluded that late-night snacking didn’t cause extra weight gain.

The difference between the studies may hold the key.

In the monkey study, only a portion of the monkeys’ food that they ate was eaten at the time that they should be sleeping. A bit like late-night snacking. But, in the mice study, their whole diet was consumed when the mice should have been sleeping.