Scientists at Harvard University have just made a startling discovery about breast cancer, finding that women who live in areas with more artificial light outside at night were more likely to get the disease. The findings were based on the Nurses’ Health Study, which followed 109,672 nurses between 1989 and 2013.

Using geocoding, they measured the average light level in neighborhoods where the nurses lived by way of satellite images from the Defense Meteorological Satellite Program. They were updated over the 15-year follow up period, during which about 3,549 new cases of breast cancer were diagnosed, which is typical for that sample size.

The study found a direct correlation between nighttime light levels form artificial sources and the risk of developing breast cancer, and the risk seemed to rise the higher the amount of light there was.

“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.