New research reveals an unexpected potential risk factor for breast cancer: city lights.

The Harvard Medical School study found an association between living in areas with high amounts of ambient nighttime light and slightly increased odds for breast cancer in younger women who smoke. “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,” study author Peter James said in a Harvard news release.

Earlier research had suggested that high levels of exposure to light at night disrupts the body’s internal clock. That might lower levels of a hormone called melanin which, in turn, might boost the risk of breast cancer. Testing the theory further, James’ group tracked almost 110,000 U.S. women, followed as part of a long-term study of nurses from 1989-2013. The researchers used nighttime satellite images and records of night swift work to help figure out the amount of nighttime light each woman might have been exposed to. The study was not designed to prove cause and effect. However, the Harvard group found that breast cancer levels in premenopausal women who currently smoked or had smoked in the past grew by 14 percent if they were in the 20 percent deemed to have had the most exposure to outdoor light at night. Furthermore, as levels of outdoor nighttime light went up, so did the likelihood of breast cancer for this subgroup of women, James’ team said.