In one study, 156 adults used a smartphone app to track what and when they ate or drank for three weeks.
"People ate around the clock," says Marie-Pierre St-Onge, associate professor of nutritional medicine at the Columbia Irving University Medical Center, who led a panel of experts convened by the American Heart Association to examine meal timing.
In fact, there were only five hours—1:00 to 6:00 a.m.—during which people ate very little.
Avoiding late-night meals may benefit your health.
How might a shorter eating window help people lose weight?
In the smartphone study, people met their calorie needs by about 6:30 p.m. "After that, you're just tacking on extra calories," says St-Onge.
What happens when people eat the same number of calories within a short or a long eating window?
To find out, researchers fed eight overweight men with prediabetes all their food during either a six-hour or a 12-hour window, starting at their usual breakfast time (between 6:30 and 8:30 a.m.).
After five weeks on each eating plan, weight loss and fasting blood sugar levels were no different.
However, the men were more insulin- sensitive and their blood pressure was roughly 10 points lower on the six-hour window plan. And they had less "desire to eat" in the evening, even though they had to stop eating by 2:30 p.m. at the latest.
Would a six-hour window later in the day work as well?
No one has looked. But a few studies have pitted early-in-the-day against late-in-the-day eating.
The "breakfast diet" front-loaded the day's intake (700 calories at breakfast, 500 at lunch, and 200 at dinner). The "dinner diet" was more typical (200 calories at breakfast, 500 at lunch, and 700 at dinner).
In another study, Iranian scientists randomly assigned 69 women who had overweight or obesity to eat 50 percent of their calories at lunch and 20 percent at dinner or vice versa. Both groups were told to divide their remaining calories between breakfast and snacks, to cut calories and fat, to eat more vegetables and fruits, and to work toward an hour of exercise five days a week.
Three months later, the "big lunch" group had lost more weight (about 12½ pounds) than the "big dinner" group (about 9½ pounds). And insulin resistance improved more in the big lunch group.
Why might it matter when you eat?
But a handful of studies isn't much. "Research on meal timing is still in the early stages," says St-Onge.
The Bottom Line: Eating light in the evening may make you less insulin-resistant and might help you shed more pounds.