If you eat right and exercise, but still can't lose weight, something as simple as outside streetlights shining in your bedroom window could be keeping you fat. New research published in Proceedings of the National Academy of Sciences found that too much artificial light interferes with the body's natural clock — its circadian rhythm — and slows fat-burning.

Scientists at Leiden University exposed mice to light 16 to 24 hours a day for five weeks. Even though they ate the same amount of food and did the same amount of exercise as a control group, they gained 50 percent more weight than the mice who kept more normal day/night hours by being exposed to light for only 12 hours.

Tests showed that being exposed to light during normal sleeping hours disrupted the ability of the body's ability to activate "brown" fat to burn calories, and the unburned calories were stored as fat.

Researcher Patrick Rensen said that even the amount of artificial light used for a quick check of email messages could disrupt the body's internal clock, and that keeping the bedroom dark can help people keep weight gain in check.

The recent findings support a human study published in 2014. A British study of 113,000 women found that the more light they were exposed to during sleeping hours, the greater their risk of being fat. Researchers found that too much night light disrupts the body's circadian rhythm and also affects metabolism.