High Levels of Outdoor Light at Night Linked to Increased Breast Cancer Risk

Women who worked the night shift found to have a greater risk of developing breast cancer.

Women who reside in areas with high levels of outdoor light at night may be at an increased risk of developing breast cancer, according to a study published in Environmental Health Perspectives.

Prior studies suggest that exposure to light at night may lead to decreased levels of melatonin, which can disrupt circadian rhythms. As a result, this leads to an increased risk of breast cancer.

In the current study, investigators examined data from nearly 110,000 women enrolled in the Nurses’ Health Study II from 1989-2013.

Data from satellite images of Earth taken at night were linked to residential addresses for each of the participants. Additionally, the investigators accounted for the influence of working the night shift, as well as health and socioeconomic factors.

The results of the study showed women exposed to the highest levels of outdoor light at night had an estimated 14% increased risk of breast cancer compared with women in the bottom fifth of exposure.

Furthermore, as levels of outdoor light at night increased, so did the rates of breast cancer.

This association was only found among women who were premenopausal and those who were current or past smokers.

Women who worked the night shift had the strongest link, suggesting both the shift and exposure to light at night contribute to breast cancer risk.

“In our modern industrialized society, artificial lighting is nearly ubiquitous,” said lead author Peter James. “Our results suggest that this widespread exposure to outdoor lights during nighttime hours could represent a novel risk factor for breast cancer.”

Although the findings are illuminating, the authors noted that more works need to be done to confirm the findings and specify potential mechanisms.

Approximately 1 of 8 women in the United States will develop invasive breast cancer in their lifetime. An estimated 252,710 new cases of invasive breast cancer are expected to be diagnosed in the United States in 2017, with 63,410 new cases of non-invasive breast cancer.