We no longer live in a world governed by the sun. Artificial light lets millions of people stay up late or work in the predawn hours. But the price many of us pay for this extra illumination is a disrupted internal clock—and, growing evidence suggests, obesity. Now, a study of mice suggests that excessive light exposure causes the rodents to burn less fat, a finding that if confirmed could lead to new paths to weight loss in humans.

Many mammals have two types of tissues that store fat: brown fat and white fat. Both store energy, but white fat releases its energy stores to power other cells, while brown fat produces heat from metabolizing its contents. For years, scientists have been trying to coax brown fat into action as a way to stimulate weight loss. They’ve identified a protein called β 3 adrenergic receptor that, when activated, encourages brown fat cells to burn off more fat and produce more heat.

To test the relationship between light exposure and brown fat activity, researchers exposed groups of mice to artificial light for 12, 16, or 24 hours per day and monitored their levels of β 3 adrenergic receptor activity. The team also monitored the rate at which energy molecules such as glucose and fatty acids were absorbed from the bloodstream by brown fat tissue to test whether the tissue was using less energy to begin with. Both metrics showed the same trend: Brown fat in mice exposed to prolonged periods of light, 16 or 24 hours compared with a normal 12, absorbed less nutrients from the blood and burned less fat as a result of reduced β 3 adrenergic receptor activity. In essence, their furnaces were using less fuel and burning less intensely. To compound the problem, the fatty molecules left in the blood stream were absorbed elsewhere—often in white adipose tissue that makes up the classical body fat that causes obesity, says team leader Patrick Rensen, a biochemist at Leiden University Medical Center in the Netherlands.

As a result, the light-bathed mice packed on between 25% and 50% more fat, despite eating and moving the same amount as the control group, the team reports online today in the Proceedings of the National Academy of Sciences. The findings may support previous work that has linked disruptions in circadian rhythm with increased obesity in both mice and humans. Some studies have even found that prolonged light exposure contributes more to obesity than a high fat diet, especially in the short term. According to Rensen, if the new study is right, a reduction in brown fat activity may account for such observations.

To help confirm that the drop off in brown fat activity was caused by disrupted circadian rhythms, the team repeated parts of their experiments after severing the nerves that connect the brown fat cells to the sympathetic nervous system—the arm responsible for sending messages that maintain biological rhythm. Without the input from the nervous system, brown fat absorbed less nutrients regardless of light exposure, indicating that, indeed, disrupted circadian signaling through the sympathetic nervous system is likely to blame for the effect on metabolism.

“I like it. I think it’s a very nice paper,” says Jörg Heeren, a biochemist at University Medical Center Hamburg-Eppendorf in Germany who was not involved in the study. “I think it’s justified to say the brown adipose tissue [brown fat] is responsible for the observed effect.”

It’s unclear whether the rodent findings will translate to humans, Rensen says, and thus it’s too soon to make any sort of medical recommendations about avoiding artificial light to lose weight or at least avoid weight gain. Still, he says that people trying to lose weight in the future might think about monitoring light exposure in addition to food. “If we can conclude anything, darkening the bedroom would be a good idea.”