Exposure to Outdoor Light at Night Linked to Breast Cancer

Outdoor light at night contributes to a host of health issues, including interrupted circadian rhythms. Recently, researchers at Harvard associated high levels of outdoor lighting at night to an increased risk for breast cancer.

Disrupted circadian rhythms can contribute to a decreased nightly secretion of melatonin, which can disturb estrogen regulation, leading to breast cancer risk. Peter James, the lead author of the study and professor within Harvard Medical School’s Department of Population Medicine, said the common use of outdoor lights at night only increases the risk.

“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,” James said in a press release.

The researchers used nationwide data from the Nurses’ Health Study II cohort and followed 109,672 women from 1989 through 2013. The study estimated overall light at night exposure by using time-varying satellite data for a composite of persistent nighttime illumination for each residence during the follow-up period.

Related: Study Shines Light on Harmful Effects of Night Shift Work

The study considered factors such as health and socioeconomic data as well as night shift work into the study. Overall, women who were exposed to the highest levels of light at night were associated with a 14 percent increased risk for breast cancer. The association between outdoor light at night and breast cancer was specifically found in premenopausal women and those who were smokers or had smoked in the past.

“This study is one of the first analyses of light at night and breast cancer that incorporates information on night-shift work,” the authors wrote. “In stratified analyses, the association between light at night and breast cancer was stronger among participants who worked night shifts at any time since 1989 than among those who did not.”

The researchers said exposure to light at night and night shift work could both contribute to increased risk for breast cancer, possibly through mechanisms involving circadian disruption. The authors cited previous studies where outdoor light at night was associated with breast cancer risk, including a study that reported a 73 percent higher incidence of breast cancer in communities with the highest light at night versus those with the lowest light at night.

Related: Light Pollution May Be Disrupting How Your Body Functions

The study authors said that while the study had limitations such as possible data processing errors, the study is strong due to its 22 years of follow-up, with time-varying and objective measures of ambient light at night across the U.S.

“[The study] provides evidence that women living in areas with high levels of outdoor LAN may be at higher risk of breast cancer even after accounting for individual and area-level risk factors for breast cancer,” the authors wrote. “Although further work is required to confirm our results and to clarify potential mechanisms, our findings suggest that exposure to outdoor light at night may contribute to breast cancer risk.”