Previous studies have shown that exposure to light at night may lead to decreased levels of melatonin, a hormone made by the body’s pineal gland. This can disrupt circadian rhythms and lead to an increased risk of breast cancer.
The study tracked data from nearly 110,000 women in the U.S. between 1989 and 2013. The results shows the risk of breast cancer in premenopausal women who smoke or who used to smoke increases with greater exposure to outdoor nighttime light. However, older women and those who had never smoked did not seem to be affected by the lighting.
“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,” Peter James, lead author of the study and assistant professor at Harvard Medical School’s department of population medicine said in a press release.
The study also finds the link was stronger among women who worked night shifts but the authors admit further work is needed to clarify the potential mechanisms.