Many of the studies involving late-night eating have been conducted on lab mice or on people who are deemed "late sleepers," not necessarily on people who routinely eat the biggest or most caloric meals late in the day.
Those who are dieting are routinely told to cut off meals after 7 or 8 p.m. to improve metabolic rates and reduce weight gain. Although it would seem to be good advice, there is no definitive scientific research that equates eating at night to weight gain.
The speculation behind the eating and weight gain connection has to do with metabolism and human evolution. Humans evolved from situations where they ate and foraged between sunrise and sunset. There were no video games to play at night, no TV?shows, and certainly no refrigerators to raid for late-night snacks. The body simply adapted to getting the bulk of its caloric intake in between daylight hours. Furthermore, people tended to be their most active during the day, when calories consumed could be burned off through exercise or daily activity. At night people tended to simply be lying around relaxing or sleeping. The calories can't be burned off.
Others debate that this is a myth, that calories consumed are simply calories. They do not weigh more if they are eaten at night or during the day.
Furthermore, the body's metabolism never stops working, organs are functioning and energy is being used even as one sleeps. Opponents to the "late night eating equals weight gain" theory state that it's not what is eaten at a specific time that matters, but it's how many calories are consumed over a week or month. As anyone who has tried to diet knows, one day of dietary changes will do little, if anything, to influence a person's weight. Weight loss is a process instead of a quick fix.
Limiting caloric intake -- no matter what time it occurs -- will help a person lose weight. That's because it limits the number of calories a person will need to burn off. Routinely eating several small meals a day can stave off hunger pangs and reduce the propensity for overeating or eating out of boredom at night. Foods consumed at night, including comfort foods, tend to be high in calories and fat anyway. Some people do find that eating a light snack about an hour before bed, such as a few crackers with cheese, can induce sleep and keep the body satiated until morning.
There's no strong evidence that eating late at night plays a role in weight gain. It's rather the number of calories and types of foods eaten as part of a normal diet.