Breast cancer incidence increases rapidly as societies industrialize. Many changes occur during the industrialization process, one of which is a dramatic alteration in the lighted environment from a sun-based system to an electricity-based system. Increasingly, the natural dark period at night is being seriously eroded for the bulk of humanity. Based on the fact that light during the night can suppress melatonin, and also disrupt the circadian rhythm, it was proposed in 1987 that increasing use of electricity to light the night accounts in part for the rising risk of breast cancer globally. Predictions from the theory include: non-day shift work increases risk, blindness lowers risk, long sleep duration lowers risk, and population level community nighttime light level co-distributes with breast cancer incidence. Thus far, studies of these predictions are consistent in support of the theory. A new avenue of research has been on function of circadian genes and whether these are related to breast cancer risk. In particular, a length variant of Per3 (5-VNTR) has been associated with increased risk in young women, and this same 5-VNTR variant has also been found to predict morning diurnal type and shorter sleep duration compared to the 4-VNTR variant. An important question is how an effect of light-at-night (LAN) exposure on breast cancer risk might be modified by polymorphisms and/or epigenetic alterations in the circadian genes, and conversely whether light-at-night exposure (e.g., shift work) can induce deleterious epigenetic changes in these genes.