If you're an owl rather than a lark, your late-night habit could be the reason you're gaining weight, say experts. And they think it's all because you're exposed to too much light.
It had nothing to do with the night mice eating more, say the experts, or being less active, since the mice all ate exactly the same amount of food and took the same amount of exercise.
So what is it about staying up late - or, rather, being exposed to light at night - that could make you put on weight?
According to the researchers, whose study has been published in the Proceedings of the National Academy of Sciences journal, it might have something to do with the fact that light disrupts levels of melatonin - a hormone you naturally produce at night.
Melatonin is made by your brain's pineal gland during darkness and is thought to help regulate the sleep-wake cycle. But melatonin is also involved in metabolism, the researchers claim.
Further tests also showed being exposed to light at night disrupted the mice's eating patterns too, which led the researchers to suggest staying up late could make you eat late. And that, they claim, may also disrupt your metabolism and make you pile on the pounds.
Have your nocturnal habits affected your health in any way?