Mice with a malfunctioning body clock piled on the pounds because they were eating when they should have been asleep, a study suggested.
Even if they consumed no more calories than normal, they still gained more weight.
The findings shed light on the complex causes of obesity in humans, said the researchers from the University of Pennsylvania.
They said the effect in mice was similar to night-eating syndrome in humans, which is associated with obesity.
The results could explain why night shift workers are more likely to suffer obesity and metabolic syndrome, a combination of diabetes, high blood pressure and obesity.
Patients with sleep disorders are also at greater risk of developing obesity, while less sleep can lead to weight gain in healthy people.
Georgios Paschos, a research associate involved in the study, said: “A relatively modest shift in food consumption into what is normally the rest period for mice can favour energy storage. Our mice became obese without consuming more calories.”