A midnight snack won't make you fatter than a midmorning munch, monkey studies show.

It's a widely accepted truism that food eaten after dark is more fattening than the same food eaten in the light of day. And there's a reason to believe this: The body does slow down at night.

Researchers have tried to look at the issue, but it's been hard to do a definitive study in humans. That's why Judy L. Cameron, PhD, senior scientist at the Oregon National Primate Research Center in Beaverton, looked at 20 female monkeys instead.

To mimic human menopause, Cameron's team removed the monkeys' ovaries. To mimic human junk food, the researchers fed the monkeys a high-fat diet. And because some of the monkeys ate about two-thirds of their calories at night, Cameron and colleagues were able to look at the effects of nighttime meals.

"The outcome was, there was no difference at all," Cameron tells WebMD. "Whether they eat by day or by night, monkeys have an equal probability of gaining weight. So weight gain depends on how many calories we eat, and not when we eat them."

When not doing monkey research, Cameron is professor of physiology and pharmacology, obstetrics and gynecology, and behavioral neuroscience at Oregon Health & Science University in Portland. She and her colleagues report their findings in the current issue of Obesity Research.

Even though they ate a high-fat diet -- nearly quadrupling their calories -- not all of Cameron's monkeys gained weight. A few got fat. Some stayed lean.

What happened?

"It was mostly due to activity level," Cameron says. "The strongest predictor of weight gain in adult monkeys was how active they were. Very active animals did not gain weight, and very sedentary animals gained quite a bit."

But these are just monkeys -- aren't they? Actually, Cameron says that monkeys are a lot like humans in terms of how they eat, sleep, and exercise.

"We think monkeys are a very, very good model for humans," she says. "As far as we can tell, their mechanisms of weight gain are the same as humans."

A real problem with obesity research is that it's almost impossible to know for sure how much human study participants actually eat and exercise. That's not a problem in monkey studies. Cameron says we can look for more information to come from her monkeys -- including studies on the impact of menopause and hormone replacement on weight.