Nighttime Light Exposure: Does It Increase Your Risk of Breast Cancer?

Women who live in areas with higher levels of outdoor light at night may be at higher risk for breast cancer than those living in areas with lower levels, according to a large long-term study from the Harvard T.H. Chan School of Public Health. The link was stronger among women who worked night shifts.


Background: Previous studies have suggested that exposure to light at night may lead to decreased levels of the hormone melatonin, which in turn can disrupt circadian rhythms – out internal “clocks” that regulate sleepiness and alertness. As a result, your risk of breast cancer may increase.

The Study: In the most comprehensive study to date (investigating possible links between  outdoor light at night and breast cancer, researchers examined data from nearly 110,000 women enrolled in the Nurses’ Health Study II from 1989-2013. The researchers linked data from satellite images of earth taken at nighttime to residential addresses for each study participant, and also considered the influence of night shift work. The study also factored in detailed information on a variety of health and socioeconomic factors among participants.

Results: Women exposed to the highest levels of outdoor light at night — those in the top fifth — had an estimated 1.14-times increased risk of breast cancer during the study period, as compared with women in the bottom fifth of exposure. As levels of outdoor light at night increased, so did breast cancer rates. The association between outdoor light at night and breast cancer was found only among women who were premenopausal and those who were current or past smokers. In addition, the link was stronger among women who worked night shifts, suggesting that exposure to light at night and night shift work contribute jointly to breast cancer risk, possibly through mechanisms involving circadian disruption.


My Take: While provocative and consistent with historic studies, there are many potential variables for which the investigators did not control. Still, while we need confirmatory studies, this may be a caution regarding night exposure. For my patients, this typically means limiting blue light exposure in the hour before bedtime. Watch out for those computer screens, cell phones, and televisions! I’m Dr. Michael Hunter.


The small print: The material presented herein is informational only, and is not designed to provide specific guidance for an individual. Please check with a valued health care provider with any questions or concerns. As for me, I am a Harvard- , Yale- and UPenn-educated radiation oncologist, and I practice in the Seattle, WA (USA) area. I feel genuinely privileged to be able to share with you. If you enjoyed today’s offering, please consider clicking the follow button at the bottom of this page.

And, one more thing: NEW free apps for Android and iOS (Apple): In apps, search My Breast Cancer by Michael Hunter.


Reference: Peter James, Kimberly A. Bertrand, Jaime E. Hart, Eva Schernhammer, Rulla M. Tamimi, Francine Laden. Outdoor Light at Night and Breast Cancer Incidence in the Nurses’ Health Study II. Environmental Health Perspectives, 2017 DOI: 10.1289/EHP935


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Harvard AB Yale MD UPenn Radiation Oncology Radiation Oncologist, Seattle area

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