Light at Night and Breast Cancer Risk

By Jacob Schor, ND, FABNO

While in Maui recently, we drove to the Haleakala Summit at 10,000 feet to watch the sunrise. We miscalculated driving times and reached the top three hours before the sun was supposed to rise. Yet we all agreed our mistake was worth it because we got to see the brightest star-studded sky any of us could remember. There wasn’t a cloud in the sky when we arrived, but by sunrise the summit was enveloped in a whiteout fog so that we never did see the sun come up or any color in the sky. I was reminded of this experience recently as I worked with Heather Wright to review a study by Portnov et al for NMJ on how exposure to light at night may affect risk of cancer.

It seems that an easy and perhaps effective way to lower risk of breast cancer may be to reduce or avoid exposure to artificial lighting at night. It’s that simple. A handful of recent studies confirm what we’ve long suspected….light pollution is a significant risk for breast cancer. Artificial light is a carcinogen.

The Portnov study reported that light at night was correlated with incidence of breast cancer in women in Connecticut.1 The researchers reported a significant association between light at night and breast cancer risk. Women living in areas with the greatest light at night had a 63% greater risk of breast cancer compared to women living in areas with the lowest light.

Blind women, because they use less electric lighting, have a lower risk of breast cancer.2 Women who work night shifts are at greater risk for breast cancer. A 2015 meta-analysis of 16 prospective cohort studies reported that increased morbidity and all-cause mortality in breast cancer corresponded with number of years engaged in night work.3 In a recently published French case-control study the risk for ER+ and PR+ breast cancer doubled, and risk for HER2+ cancers nearly tripled in women working nights shifts.4

The first time I noticed this idea of measuring light at night was in Kloog’s 2008 study. They used NASA nighttime satellite photos to estimate the LAN levels in Israel and reported a strong association with breast cancer rates, a 73% higher breast cancer incidence in the bright communities compared to dimmer communities.5

Something as simple as closing shutters or drawing the blinds at bedtime may reduce breast cancer risk by 18%.6

Given the weight of the evidence we need to start treating light at night as a carcinogenic environmental pollutant and strive to reduce exposure. There is a fascinating website devoted to this mission. The International Dark Sky Association advocates for preserving darkness at night and encourages communities to preserve and protect dark sites through responsible lighting policies and public education.

They also provide some fascinating images of the world as seen from space at night. If you are not familiar with this group, take a moment to locate your home and see what it looks like at night.

About the Author

Jacob Schor, ND, FABNO, is a graduate of National University of Naturopathic Medicine, Portland, Oregon, and recently retired from his practice in Denver, Colorado. He served as president to the Colorado Association of Naturopathic Physicians and is a past member of the board of directors of the Oncology Association of Naturopathic Physicians and American Association of Naturopathic Physicians. He is recognized as a fellow by the American Board of Naturopathic Oncology. He serves on the editorial board for the International Journal of Naturopathic Medicine, Naturopathic Doctor News and Review (NDNR), and Integrative Medicine: A Clinician's Journal. In 2008, he was awarded the Vis Award by the American Association of Naturopathic Physicians. His writing appears regularly in NDNR, the Townsend Letter, and Natural Medicine Journal, where he is the past Abstracts & Commentary editor.


  1. Portnov BA, Stevens RG, Samociuk H, Wakefield D, Gregorio DI. Light at night and breast cancer incidence in Connecticut: An ecological study of age group effects. Sci Total Environ. 2016 Dec 1;572:1020-1024.
  2. Stevens RG. Working against our endogenous circadian clock: Breast cancer and electric lighting in the modern world. Mutat Res. 2009 Nov-Dec;680(1-2):106-8.
  3. Lin X, Chen W, Wei F, Ying M, Wei W, Xie X. Night-shift work increases morbidity of breast cancer and all-cause mortality: a meta-analysis of 16 prospective cohort studies. Sleep Med. 2015 Nov;16(11):1381-7.
  4. Cordina-Duverger E, Koudou Y, Truong T, et al. Night work and breast cancer risk defined by human epidermal growth factor receptor-2 (HER2) and hormone receptor status: A population-based case-control study in France. Chronobiol Int. 2016;33(6):783-7.
  5. Kloog I, Haim A, Stevens RG, Barchana M, Portnov BA. Light at Night Co-distributes with Incident Breast but not Lung Cancer in the Female Population of Israel. Chronobiol Int. 2008;25(1):65-81.
  6. Keshet-Sitton A, Or-Chen K, Yitzhak S, Tzabary I, Haim A. Can Avoiding Light at Night Reduce the Risk of Breast Cancer? Integr Cancer Ther. 2016 Jun;15(2):145-52.