They also saw a stronger link among women who work at night.
"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," lead author Peter James said in a statement.
Why is that?
Light affects melatonin, a hormone that regulates sleep-wake cycles and plays a role in reducing tumor growth. However, exposure to artificial light lowers melatonin levels, preventing it from operating at its full ability.
Researchers noted that the association between outdoor light at night and breast cancer was only prevalent among premenopausal women and current or past smokers.
They also acknowledged that more research needs to be done to clarify their results and methods.