Invited commentary: nighttime light exposure as a risk factor for obesity through disruption of circadian and circannual rhythms.

In this issue of the American Journal of Epidemiology, McFadden et al. (Am J Epidemiol. 2014;180(3):245-250) report findings on the relationship between light exposure at night and obesity from a cross-sectional study of United Kingdom women. Their research extends findings from a previous study with elderly participants by including a larger sample size of over 100,000 women and a broader age range of 16 years or older. The findings are consistent with animal studies showing that prolonged light exposure leads to weight gain. Humans' circadian, circannual, and metabolic regulatory systems evolved to be adaptive in environments that were quite different from those faced in modern industrial society. Technology has allowed exposures to levels and timing of light, nutrient intake, and physical activity never before possible. This commentary discusses how nighttime light exposure can increase the risk of obesity and the metabolic syndrome by disrupting circadian and circannual rhythms.