Watch out for those late-night snacks, they may be more likely to stick around, according to a study published online in the journal Obesity.

The study was the first to provide evidence for the long-suspected relationship between eating at the “wrong” time and weight gain.

Researchers from Northwestern University interested in how meal time affects weight gain looked to lab mice for some answers.

They gave one group of nocturnal mice high-fat food during their normal waking hours. Another group of mice got the same food during the daylight hours when they would normally be getting some shuteye. The mice that simulated human late-night snacking—supping when they would be naturally sleeping—packed on a 48 percent gain in body weight. The mice on a regular schedule gained an average 20 percent.

Meal times are what tipped the scales, say the researchers. The mice that only ate during their natural sleeping hours gained more than double the weight of those that dined exclusively during their regular waking hours.

The mice got similar amounts of food and exercise, though the authors do note that the mice that ate during their typical sleeping hours consumed slightly more food and exercised less. Lead author Deanna Arble, a doctoral student working at Northwestern’s Center for Sleep and Circadian Biology, says even with that in mind, “In the end the only thing that was altered was the time of day they ate, and the results show that leads to weight gain.”

In both mice and humans, biological clocks regulate the 24-hour cycles of each day, aligning sleep, activity and bodily functions. Other recent studies have linked animals’ energy regulation and circadian rhythms, so the researchers speculated that timing of meals may make a difference in how quickly we burn off calories.

Albert Stunkard, M.D., an expert on human night eating and weight gain, and the director emeritus of the University of Pennsylvania Center for Weight and Eating Disorders, is “very enthusiastic” about the findings. “In the past, it’s been very difficult to provide evidence of this pattern in humans because of small errors in human recall about how much was eaten,” he says, “but mice can be followed with close observation.”

Although waking up and grabbing some late-night snacks isn’t a good idea, Arble says swearing off evening munchies won’t automatically lead to weight loss. “The field is still too young to go prescribing correct times to eat for an individual,” she says.

Dina Fine Maron covers health, science and politics.