Does This Light Make Me Fat?

In 2003, a group of Italian university scientists and public health officials began sifting through the health records of shift workers at a factory in Apulia, Italy. The scientists had designed a cross-sectional study of 319 male workers with normal glucose and insulin levels, and were looking for metabolic and cardiovascular risk factors. Their study, published in the International Journal of Obesity, showed that workers on the night shift at the factory were significantly more likely to gain weight and show increases in systolic blood pressure than day workers.

“The prevalence of obesity was higher among shift workers compared to day workers,” the researchers found, “whereas body fat distribution was not different between the two groups.” The investigators concluded that “shift working was associated with BMI, independently of age and work duration.”

We know that diet, calories, and exercise are key determinants of obesity. But too much food and a lack of physical activity aren’t the whole story, as genetic studies have begun to demonstrate. And now, evidence is mounting that environmental factors play a role as well. One of the puzzling aspects of obesity is the tendency for some overweight people to snack heavily on high-carbohydrate foods at night. Years ago, Richard and Judith Wurtman, a husband and wife research team at the Massachusetts Institute of Technology (MIT) warned that the consumption of excessive snack carbohydrates at night might be “leading to severe obesity” in a form that very much resembled “a kind of substance abuse.”

Light is what powers our circadian clocks, and prior studies with mice have shown that mutations in so-called “clock” genes make the animals more susceptible to obesity and other metabolic disturbances. Laura Fonken, Joanna Workman, and others at Ohio State University and at the University of Haifa in Israel undertook a study of mice designed to determine the relationship between weight gain and extra light at night. Writing in the Proceedings of the National Academy of Sciences, the researchers found that mice housed for eight weeks under either bright lights or dim lights during the night “have significantly increased body mass and reduced glucose tolerance compared with mice in a standard light/dark cycle, despite equivalent levels of caloric intake and total daily activity output.”

Neuroscientist Laura Fonken told DiscoveryNews, “With the advent of electrical lighting at the turn of the 20th century, individuals of many species, including humans, became exposed to bright and unnatural light at night.” This technological development, Fonken and others believe, may be one of the driving forces behind epidemic levels of obesity in the U.S. and elsewhere.

What Fonken and the others had discovered was that “nighttime illumination at a level as low as 5 lx is sufficient to uncouple the timing of food consumption and locomotor activity, resulting in metabolic abnormalities.”

The study drew a blizzard of press coverage, in which the tempting leap from mouse studies to human eating behavior was undertaken at a dead run. How much can this really matter to humans? Potentially, quite a bit. As the researchers note, even brief circadian misalignment in humans can lead to “adverse metabolic and cardiovascular consequences.”

“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 of Ohio State, a co-author of the study. Here’s what happened: The light-at-night mice, normally nocturnal feeders, switched partially to day feeding. “When we restricted their food intake to times whey they would normally eat, we didn’t see the weight gain,” according to Fonken. Eating at odd times in the circadian cycle disrupted the metabolism of the light-at-night mice — just as it disrupted the metabolism of the Italian shift workers — and the mice gained weight. “This further adds to the evidence that the timing of eating is critical to weight gain,” Fonken said.

Co-author Nelson summed up the results: People who stay up late and eat at night “may be eating at the wrong times, disrupting their metabolism. 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.”


Arble DM, Bass J, Laposky AD, Vitaterna MH, & Turek FW (2009). Circadian timing of food intake contributes to weight gain. Obesity (Silver Spring, Md.), 17 (11), 2100-2 PMID: 19730426

Di Lorenzo L, De Pergola G, Zocchetti C, L’Abbate N, Basso A, Pannacciulli N, Cignarelli M, Giorgino R, & Soleo L (2003). Effect of shift work on body mass index: results of a study performed in 319 glucose-tolerant men working in a Southern Italian industry. International journal of obesity and related metabolic disorders : journal of the International Association for the Study of Obesity, 27 (11), 1353-8 PMID: 14574346

Fonken LK, Workman JL, Walton JC, Weil ZM, Morris JS, Haim A, & Nelson RJ (2010). Light at night increases body mass by shifting the time of food intake. Proceedings of the National Academy of Sciences of the United States of America, 107 (43), 18664-9 PMID: 20937863

Ha M, & Park J (2005). Shiftwork and metabolic risk factors of cardiovascular disease. Journal of occupational health, 47 (2), 89-95 PMID: 15824472

Dirk Hanson, MA

Dirk Hanson, MA, is a freelance science writer and the author of "The Chemical Carousel: What Science Tells Us About Beating Addiction." He is also the author of ''The New Alchemists: Silicon Valley and the Microelectronics Revolution.'' He has worked as a business and science reporter for numerous magazines and trade publications. He currently edits the Addiction Inbox blog, and is senior contributing editor for the addiction and recovery website, The Fix.
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