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Late-night forays to the fridge might have you packing on the pounds even faster than munching during the day does, a new mouse study suggests.

That's because the body's internal clock, or circadian rhythm, may play a role in metabolism, researchers say.

"We have found causal evidence that eating during the 'wrong' circadian time leads to weight gain in mice," said lead researcher Deanna Arble, a doctoral candidate in Northwestern University's Center for Sleep and Circadian Biology.

"While I do not believe the field is advanced enough to prescribe appropriate eating times for each individual, I believe we can at least say that humans should avoid eating during their normal sleeping phase because this could lead to increased weight gain," she said.

The report is published in the Sept. 3 online edition of Obesity.

Over six weeks, Arble's team fed two groups of mice a high-fat diet. The animals were allowed to eat as much as they wanted. Some mice were fed during the day -- normal sleeping time for mice - while others were fed at night, when they are typically most active.

The researchers found that changing the feeding time, by itself, affected the rodents' body weight. Mice fed during their normal sleeping hours gained more weight than mice that ate during their typical wakeful hours.

In fact, mice that ate during normal sleeping hours gained 48 percent more weight over their weight at the start of the study, while the animals fed during normal wakeful hours gained only 20 percent more weight.

The mice fed during their normal sleep-time also had an almost 8 percent higher level of fat as a percentage of overall weight, the team said.

All of the mice took in the same number of calories and expended about the same amount of energy, although the mice that ate at the "wrong" time had somewhat less activity, the researchers noted.

"We have demonstrated that mice eating at the 'wrong' time of day have increased weight gain compared to mice eating at the 'right' time of day," Arble said.

Similar to that of the mouse, humans' internal clock governs daily cycles of feeding, activity and sleep. Recent studies have shown that the body's internal clock also regulates energy use, which suggests that the timing of meals may make a difference in balancing caloric intake and energy expenditure, the researchers say.

But it is also important to not lose sight of the importance of total caloric intake, Arble said.

"If you are taking in excess calories daily, the time you eat probably doesn't matter -- you will still gain weight," she said. "Similarly, if by eating small meals for dinner you decrease your overall caloric intake, that could be more beneficial than timing. However, for the individual who is not consuming excess calories and is still gaining weight, this experiment in mice suggests a new factor to examine -- the timing of feeding."

Dr. Luigi F. Meneghini, an associate professor of clinical medicine at the Diabetes Research Institute of the University of Miami Miller School of Medicine, believes the level of activity of the mice may be one reason for the difference in weight gain.

Meneghini noted that the mice fed at the "wrong time" exercised less than the mice fed during the normal wakeful hours. "Maybe something happens with circadian rhythms or hormones that make it less likely that they will engage in physical activity," he said.

"Based on this small study, if one were to say is it caloric intake or physical activity that led to the difference in weight gain, one would surmise it was more likely physical activity," he said.

More information

For more information on obesity, visit the U.S. National Library of Medicine.

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