A new study out of Harvard University has found that there is a greater risk of breast cancer in women who live in neighborhoods with lots of artificial light outdoors at night. Researchers used the Nurses’ Health Study to come to their conclusions, following nurses in the NHS for breast cancer occurrence between 1989 and 2013.

They geocoded the home of 109,672 nurses, and estimated the average light level in the neighborhood at night with satellite images taken by the Defense Meteorological Satellite Program, which were updated over the 15-year follow up period.

They diagnosed 3,549 new cases of breast cancer by 2013, which is a typical percentage. The study found that there seemed to be a direct relationship between nighttime light level and the risk for developing breast cancer, and it appears that the higher that light level is, the greater the risk.

“A large long-term study found that breast cancer risk may be higher for women who live in areas with high levels of outdoor light at night,” the Harvard statement reads. “The link between outdoor light at night and breast cancer was found only among women who were premenopausal and were current or past smokers, and was stronger among those who worked night shifts.”

“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 at Harvard Pilgrim Health Care Institute, who did the work while a research fellow in the Departments of Epidemiology and Environmental Health at Harvard Chan School.