Midnight snacking piles on the pounds, warns new research.
Eating during the day - instead of in the evening - is the key to shedding weight, according to the study.
People who have a meal at night don't lose weight - even when they are on a diet, suggests the research.
That is because they don't burn off the calories when inactive as the body's metabolism is regulated by a person's biological - or circadian - clock.
In experiments, mice on a reduced calorie plan that ate during their normal cycle of feeding - during the time of day they are active - were the only ones among five groups to lose weight.
That was despite consuming the same amount as another group fed during their rest time - which in the nocturnal animals is daylight.
Dr Joseph Takahashi, a neuroscientist at Texas University in Dallas, said: "Translated into human behaviour these studies suggest dieting will only be effective if calories are consumed during the daytime when we are awake and active.
"They further suggest eating at the wrong time at night will not lead to weight loss - even when dieting."
The findings, published in the journal Cell Metabolism, add to growing evidence that WHEN people eat is as important as what they eat.
They follow a study by British scientists that found people who eat late in the evening are less healthy.
Modern lifestyles mean evening meals are getting later - and metabolism slows down during the hours of darkness.
Dr Takahashi and colleagues used a new high-precision system to reinforce the idea the time of day food is eaten is more critical to weight loss than the amount of calories ingested.
High-tech sensors and automated feeding equipment also showed why calorie-restricted diets improve longevity. They say the new set of tools has offered fresh insights.
Mice on a diet reduced their eating to a very short time period and were unexpectedly active during the day - their normal rest period.
The figures reveal previously unknown relationships among feeding, metabolism, and behaviour, according to the researchers.
Dr Takahashi said: "It has been known for decades caloric restriction prolongs lifespan in animals but these types of studies are very difficult to conduct because they required manual feeding of subjects over many years.
"Therefore shortcuts were taken in order to deal with practical matters such as the normal Monday-to-Friday work week."