Now, in a new study, researchers from Northwestern University have found that eating at the "wrong" time leads to more than twice as much weight gain, even when the overall calories consumed are the same as those eaten at appropriate times.
Their research is confined to animals, but the results are dramatic enough to point to the need for more human research, says Deanna Arble, a PhD student at Northwestern and the study's lead author.
''We've found that mice who are allowed to eat during the light phase -- their 'wrong' time of day -- gain substantially more weight than those allowed to eat during the dark phase, the right time of day for them to eat," she tells WebMD. The study is published online in the journal Obesity.
Based on the research, however, it's not possible, Arble says, to set an optimal time window for people to eat to maintain weight. Rather, she hopes the finding will be a trigger for obesity scientists who study people to focus more closely on the concept of the timing of eating.
In the study, Arble and her colleagues gave two groups of mice, who are nocturnal and expected to eat at night, the same high-fat diet. They gave one group access to food at night and the other group access during the day. Both groups could eat as much as they wanted during the 12-hour feeding phase.
'Right' Time vs. 'Wrong' Time
At the end of the six-week study, the mice who were fed during the light phase -- their ''wrong'' time to eat -- gained much more weight than those fed during the dark phase.
When the researchers compared the animals' weight at the study start to their weight at the end, the mice that ate at the wrong time had a 48% weight increase, while those who ate at the correct time had a 20% weight increase.
While both groups gained, Arble notes, the mice that ate at the wrong time gained more than twice as much weight. "We did not restrict the amount of calories they were eating," she says. Even so, between groups, ''there was no difference in the [average] amount of calories consumed."
The only variable, she says, was when the food was consumed.
Arble can't say for sure why the mice that ate at the ''wrong'' time gained so much more weight. ''We speculate that it's the interplay between body temperature, metabolic hormones such as leptin, and the sleep-wake cycle," she says.
For humans, nighttime is a time for rest, as the body temperature declines, she says. "Eating at night is contradicting your body's natural circadian rhythm,'' she says. "The leptin levels are starting to rise, and are supposed to be discouraging you from eating." Rising leptin levels suppress appetite.
She is hopeful that other researchers will focus on the same concept in human studies. "If it turns out to be a major factor,'' she says of the food timing, ''it will be a great way to help weight maintenance and maybe weight loss. It would be a fairly simple behavior modification, to move the time you are eating."
But night-time eating would be one factor among many contributing to weight gain, Arble tells WebMD. "I don't want people to read this study and think 'Oh, I can eat as much as I want as long as it's the right time of day.'"
''Eating too much late at night is not good," says Arline D. Salbe, PhD, a senior research fellow at the Kronos Longevity Research Institute in Phoenix, who has also researched and published on the topic.
''In a simple but elegant study design using mice, Arble [and her colleagues] have confirmed our own results in humans that nighttime eating is a risk for weight gain," says Salbe. "As modern lifestyles continue to modify work and sleep patterns, the risk of weight gain from nighttime energy intake becomes more relevant to greater numbers of people," she tells WebMD.
In her own study, Salbe and her colleagues evaluated the food intake of 94 people over a three-day period while they stayed in a clinical research unit and were allowed to eat as much as they wished. They found that 29 were night eaters, defined as those who ate between 11 p.m. and 5 a.m. on at least one of the three days; 65 were not night eaters.
When they followed them for nearly 3.5 years, the night eaters gained 13.6 pounds while the non-night eaters gained 3.7.
The nighttime eating, Salbe found, predicted weight gain. Her study is published in The American Journal of Clinical Nutrition.
Her advice? "I think the healthiest way to maintain weight is to eat breakfast, lunch, and dinner," Salbe says. "If dinner is early enough and you stay up late enough, having a snack at 8 or 9 o'clock is perfectly fine." By snack, she says, she means a small amount of food, not an overflowing plate of pasta.