Why? Because being exposed to higher levels of artificial light at night decreases levels of the hormone melatonin, which puts women at a greater risk of breast cancer.

Women who live in areas with higher levels of outdoor light at night may be at higher risk for breast cancer than those living in areas with lower levels, according to a large long-term study from Harvard T.H. Chan School of Public Health. The link was stronger among women who worked night shifts. The study was published in Environmental Health Perspectives.

“In our modern industrialized society, artificial lighting is nearly ubiquitous. Our results suggest that this widespread exposure to outdoor lights during nighttime hours could represent a novel risk factor for breast cancer,” said lead author Peter James, assistant professor at Harvard Medical School’s Department of Population Medicine. Previous studies have suggested that exposure to light at night decreases levels of the hormone melatonin, which can disrupt circadian rhythms – our internal “clocks” that govern sleepiness and alertness - and, in turn, increase breast cancer risk.

The new study, the most comprehensive to date to examine possible links between outdoor light at night and breast cancer, looked at data from nearly 110,000 women enrolled in the Nurses’ Health Study II from 1989-2013. The researchers linked data from satellite images of Earth taken at night-time to residential addresses for each study participant, and also considered the influence of night shift work. The study also factored in detailed information on a variety of health and socio-economic factors among participants. Women exposed to the highest levels of outdoor light at night - those in the top fifth - had an estimated 14% increased risk of breast cancer during the study period, as compared with women in the bottom fifth of exposure, the researchers found. As levels of outdoor light at night increased, so did breast cancer rates.