Too much light at night appears to lead to weight gain, according to a report, which may provide fresh clues on obesity.
The researchers found that mice exposed to a dim light at night over eight weeks had a weight gain that was about 50% more than other mice that lived in a standard light-dark cycle.
"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," says Laura Fonken, lead author of the study and a researcher at Ohio State University.
The study, which appears in the Proceedings of the National Academy of Sciences, says the weight gain could be a sign that light has an effect on the metabolism.
"Something about light at night was making the mice in our study want to eat at the wrong times to properly metabolize their food," says Dr Randy Nelson, co-author of the study and professor of neuroscience and psychology at Ohio State.
If these results are confirmed in humans, it would suggest that late-night eating might be a particular risk factor for obesity, Nelson says.
In one study, mice were housed in one of three conditions: 24 hours of constant light, a standard light-dark cycle that included 16 hours of light and eight hours of dark, or 16 hours of daylight and eight hours of dim light.
Results showed that, compared to mice in the standard light-dark cycle, those in dim light at night had significantly higher increases in body mass, beginning in the first week of the study and continuing throughout.
By the end of the experiment, dim-light-at-night mice had gained about 12 grams of body mass, compared to eight grams for those in the standard light-dark cycle.
Mice in constant bright light also gained more than those in the standard light-dark cycle, but Nelson says the mice exposed to dim light was a more important comparison for humans.
Although the mice exposed to dim light did not eat more than others, they did change when they ate, consuming more food at night.
Since the timing of eating seemed significant, the researchers did a second study, with a change: instead of having food freely available at all times, food was restricted to either the times when mice would normally be active or when they would normally be at rest.
In this experiment, mice exposed to the dim light at night did not have a greater gain in body mass than did the others when their food was restricted to times when they normally would be active.
"When we restricted their food intake to times when they would normally eat, we didn't see the weight gain," says Fonken. "This further adds to the evidence that the timing of eating is critical to weight gain."
The researchers say the findings offer clues for causes of the obesity epidemic in Western countries.
"Light at night is an environmental factor that may be contributing to the obesity epidemic in ways that people don't expect," says Nelson. "Societal obesity is correlated with a number of factors including the extent of light exposure at night."
He says prolonged computer use and television have previously been linked to obesity, but because they are associated with a lack of physical activity.
"It may be that people who use the computer and watch the TV a lot at night may be eating at the wrong times, disrupting their metabolism," says Nelson.
"Clearly, maintaining body weight requires keeping caloric intake low and physical activity high, but this environmental factor may explain why some people who maintain good energy balance still gain weight."