Prolonged exposure to artificial light may be a better predictor of weight gain than diet, and this phenomenon goes back to prehistoric times. Carbs, genetics, “bad” fats — there are many theories about the causes of obesity and associated illnesses like diabetes and heart disease. But new research in today’s Proceedings of the National Academy of Sciences concludes obesity is not just about food. Sander Kooijman and colleagues at the Leiden University Medical Center in the Netherlands say that weight gain may be increasingly common in today’s fast-paced global economy because prolonged artificial light exposure actually inhibits the fat-burning processes that normally occur during darkness. “The modern 24-hour economy necessitates work at night and shifts social activities to the dark hours,” study co-author Patrick C. N. Rensen explained. “Our observations may implicate, assuming the data could be extrapolated to humans, that the current obesity epidemic is at least partly due to increased light pollution.” Several years of research correlates weight gain with artificial light exposure, which is defined as any light that isn’t from the sun, including overhead lighting, computer screens, and streetlights. A 2013 article in the Federation of American Societies for Experimental Biology journal found that mice fed a low-fat diet but exposed to artificial light for prolonged periods of time actually gained more weight than mice fed a diet high in fat. Get the Facts: What Is Obesity? »

Why Would Light Make Us Fat? Kooijman’s team wanted to know more about what exactly caused the correlation, so they looked at the exact mechanism behind that weight gain — specifically a type of fat known as brown adipose tissue (BAT). This tissue plays a central role in energy expenditure by changing energy from food into heat. In short, it burns calories. Researchers exposed mice to artificial light for 12, 16, or 24 hours per day for five weeks. Mice exposed to artificial light for 24 hours, compared with 12 hours, had significantly higher fat composition despite consuming the same diet. Further investigation revealed that BAT activity was decreased by light exposure. As mice were exposed to longer periods of light, conversion of fatty acids and glucose into heat was reduced. The longer the light shined, the fewer calories the mice burned. In a culture where sunny days evoke swimsuits and beach bodies, it’s hard to understand why light exposure would lend itself to the deposition of fat. But the body’s circadian rhythms are uniquely attuned to variations in light and dark. Disruption of these basic physiological processes can have significant metabolic repercussions. Researchers think that exposure to artificial light is actually tricking an evolutionary adaptation that encouraged our ancestors to store fat in the summer months to prepare for the colder, darker winter months. “The so-called ‘day length’ prepares the body for cold adaptation due to seasonal variation,” said Rensen. Shortened days, he explains, usually increase BAT activation to create heat in the body to prepare for cold, whereas longer days don’t require the creation of heat — so the body stores the excess energy, or fat, until it is needed. “Lowering BAT activity in warm seasons may indeed result in beneficial storage of fat,” Rensen said. But artificial light confuses the body. A night shift worker in a hospital, for example, might spend 12 hours under fluorescent lights and then several daylight hours running errands. If this happens multiple times a week, the body might adapt to the prolonged light exposure by reducing its normal fat-burning rate. Read More: Doctors Finally Getting Around to Treating Obesity »