LOS ANGELES, Oct. 11 (Xinhua) -- Exposure to too much light at night may lead to obesity, even without changing physical activity or eating more food, according to a new study based on a research on mice.
Researchers at Ohio State University said the findings show that mice exposed to a relatively dim light at night over eight weeks had a body mass gain that was about 50 percent more than other mice that lived in a standard light-dark cycle.
Mice living with light at night eat at times they normally wouldn't, the researchers explained.
If food availability was restricted to normal eating times, mice exposed to light at night gained no more weight than did mice in a normal light-dark cycle, the study showed.
"Something about light at night was making the mice in our study want to eat at the wrong times to properly metabolize their food," said Randy Nelson, co-author of the study and a professor of neuroscience and psychology.
In the research, mice were housed in one of three conditions: 24 hours of constant light, a standard light-dark cycle (16 hours of light at 150 lux, 8 hours of dark), or 16 hours of daylight and 8 hours of dim light (about 5 luxes of light).
The researchers measured how much food the mice ate each day. They also measured how much they moved around their cages each day through an infrared beam crossing system. Body mass was calculated each week.
Results showed that, compared to mice in the standard light-dark cycle, those in dim light at night showed significantly higher increases in body mass, beginning in the first week of the study and continuing throughout.
By the end of the experiment, light-at-night mice had gained about 12 grams of body mass, compared to eight grams for those in the standard light-dark cycle.
The dim light-at-night mice also showed higher levels of epididymal fat, and impaired glucose tolerance - a marker of pre- diabetes.
"Although there were no differences in activity levels or daily consumption of food, the mice that lived with light at night were getting fatter than the others," said Laura Fonken, lead author of the study and a doctoral student in neuroscience.
If these results are confirmed in humans, it would suggest that late-night eating might be a particular risk factor for obesity, Nelson said.
The findings were published online in the Proceedings of the National Academy of Sciences on Monday.