Researchers at the University of Pennsylvania say they've created a special kind of mouse that offers clues to nighttime eating and why that behavior can make us pile on the pounds.

There's a master clock in the brain that orchestrates many body functions. About 10 years ago, scientists discovered that individual cells also have their own clocks, circadian rhythms that regulate liver, muscle and fat cells.

Geneticist Steve Kay, dean of the Dornsife College of Letters at the University of Southern California, reviewed the study published in the journal Nature Medicine.

Kay says the Philadelphia team found evidence that there's a two-way conversation between the brain clock and the timers in individual cells. "What we didn't realize is that the peripheral tissues are also signaling back to the brain clock," Kay said.

Garret FitzGerald, director of Penn's Institute for Translational Medicine and Therapeutics, led the researchers who tinkered with -- and broke the clocks -- in the fat cells of mice.

"He removed that molecular clock only in fat cells," Kay said. "And then he asked the question: What happens when fat cells don't have a 24-hour rhythm, don't have this clock gene?"

The mice got fat.

"But they didn't get fat because they ate more calories, or exercised less," FitzGerald said. "They got fat because they displaced the timing of when they consumed their calories."

The mice ate 20 percent of their calories during their rest time, kind of like humans hitting the fridge at midnight.

So why did this happen? FitzGerald said his team found disordered cycling in certain genes in the brain's hypothalamus.

"At the time of inappropriate feeding, the genes that were likely to make you want to feed yourself were increased," FitzGerald said. "And the genes that would make you want to starve yourself were decreased."

Experts say this fundamental discovery in mice, if it turns out to be true for humans, could give doctors a new tool for managing and treating obesity and conditions such as diabetes.

"I'm not saying total calories isn't important, but the timing of when you consume these calories is also important," FitzGerald said.