Eating late at night does not make you fat, according to a study.
A calorie is a calorie at any time of the day, say experts
Scientists at Oregon Health & Science University in the United States carried out tests on 47 female monkeys.
They found no link between when the animals ate and whether or not they put on weight.
Speaking at a Society for Neuroscience meeting in New Orleans, the scientists said claims that eating late lead to weight gain may be "an urban myth".
Dr Judy Cameron and colleagues came across the finding almost by accident.
Their study was originally designed to find out more about the relationship between female hormones and weight gain.
As part of the study, they surgically removed ovaries from 19 of the 47 monkeys.
Removing the ovaries causes levels of the female hormones estrogen and progesterone to drop, in much the same way as happens with the menopause.
The scientists found that monkeys who had their ovaries removed started to eat more and put on weight.
"The absence of these hormones resulted in a 67% jump in food intake and a 5% jump in weight in a matter of weeks," said Dr Cameron.
They also found that levels of a hormone called leptin increased in these monkeys. Leptin is produced by fat cells and is known to influence food intake.
Previous studies have found that many women start to put on weight during menopause. The scientists believe their findings could lead to treatments to help tackle that problem.
During the study, the scientists also found the monkeys ate their food at different times of the day. Many ate mostly in the evenings and at night.
However, perhaps unexpectedly the researchers found no link between when the animals ate and how much weight they put on.
"Time and again, we've been told that eating late at night should be avoided because it will cause weight gain," said Dr Cameron.
"However, there isn't a lot of research to back up this commonly held belief, which may in fact be somewhat of an urban myth.
"In conducting this study, we noted the times that animals ate. Some of the monkeys ate most of their food during the evening and night-time hours.
"However, weight gain and the time of day that the animals were feeding had no correlation whatsoever."
Nigel Denby of the British Dietetic Association backed the findings.
"The bottom line is a calorie is a calorie whenever you eat it," he told BBC News Online.
"Your body doesn't really recognise what time of day it is. It is a little bit of a myth.
"I think it dates back to when people started taking an interest in diets. People are more likely to eat fatty snacks at night when they are watching television. In order to try to avoid that, they put restrictions on when they should eat.
"Certainly, if you need to eat after 6pm or 8pm it is probably healthier to sit down and have a meal. Otherwise, you risk grazing on fatty foods."