Late Night Eating Linked to Weight Gain

By By Tara Parker-Pope

Published: September 3, 2009

A new study in mice suggests that it's not just how much you eat, but when you eat it, that influences weight gain.

Researchers at Northwestern University wanted to test whether the timing of meals could influence body weight. Many diet books advise would-be weight losers to stop eating after 6 or 7 p.m. However, it’s never been clear if the strategy works as a behavioral change — we tend to overeat in the evenings in front of the television and the computer. Or is there some physiological reason late-night eating adds extra pounds?

To test whether time of feeding alone can affect body weight, the researchers studied two groups of mice who were fed identical diets of food that contained 60 percent fat. Mice are nocturnal, and they typically consume the vast majority of their calories at night and sleep during the day. For the study, half the mice were fed the diet during the daylight hours when they would normally be sleeping — simulating late-night eating in humans. The other half were given the same food on their regular eating schedule.

At the end of the six week study period, mice in both groups had consumed about the same amount of calories and performed the same amount of exercise. However, the mice who ate when they normally would have been sleeping hours posted an average 48 percent increase in body weight. The mice who ate on a regular schedule had an average increase of 20 percent of body weight. The findings will be published in the October issue of the journal Obesity.

Fred Turek, director of the Center for Sleep and Circadian Biology at Northwestern and the study’s senior author, said that human studies are needed to determine if timing of food intake influences body weight, but the study suggests that late-night eating may be worse, in terms of weight gain, than eating during normal waking hours. The findings would be particularly important for shift workers, who are known to be at higher risk for obesity, diabetes and other health problems. But he notes that it’s not just shift workers who are eating late. Most people eat a large percentage of their calories in the evening and continue eating late into the night.

Dr. Turek notes that humans evolved from a situation where they ate and foraged between sunrise and sunset. “After sunset, there were no refrigerators, no food just hanging around,” he said. “You didn’t eat. But today, people eat most of their calories after sunset.”