Light at Night and Breast Cancer Risk 2017

Jacob Schor ND, FABNO

March 13, 2017



While in Maui to attend the Doc Talks Conference in early March, we woke Dr. Mark Davis up from a sound sleep and got him to drive us to the top of the island, to the Haleakala Summit at 10,000 feet, just to watch the sunrise.  We miscalculated driving times and reached the top way too early, at about 4:30 AM, some three hours before the sun was supposed to rise. Yet we all agreed our mistake was well worth the loss of sleep because we got to see the brightest star studded sky any of us could remember.  Though there wasn’t a cloud in the sky when we arrived, by sunrise the summit was enveloped in a whiteout fog so that we never did see the sun come up or even any color in the cloud.  Living in the middle of Denver, it is hard to remember just how dark night is supposed to be.  That Hawaiian peak was dark beyond description and the starts were well like stars in the sky, beyond count. This experience of the night time sky has been on my mind as I worked with Heather Wright recently to review a recent study on how exposure to light at night may affect risk of cancer for NMJ.


It seems that the easiest and perhaps most 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 lights are a carcinogen. 


In December 2016 Portnov and colleagues reported that light at night was correlated with incidence of breast cancer in women in Connecticut. The study asked whether women living in communities with brighter nighttime outdoor lights had higher breast cancer incidence rates.

 The researchers found 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.


This study is one of a number of similar studies from around the world that report a  similar correlation. The findings of these studies are consistent enough to suggest that simply living in an environment that is darker at night may lower risk of breast cancer.


This isn’t a new concern. Richard Stevens first suggested that electric lighting might alter breast cancer risk in 1987.   He proposed that because light lowers melatonin production, which in turn increases estrogen levels, that the increased estrogen in turn might increase breast cancer risk.  


Though Stevens’ idea has been around for decades it has been difficult to prove;  few people live with out electric lighting these days to serve as a control group. Yet, several predictions based on Stevens’ theory have been tested, proven and appear to confirm the hypothesis. 


The first prediction was that blind women, because they use less electric lighting would have a lower risk of BC.  They do.  The second prediction was that women who work night shifts would be at greater risk for breast cancer. A number, though not all, studies have found this to be true.         A 2015 meta-analysis of 16 prospective cohort studies, reported increased that morbidity and all-cause mortality in breast cancer corresponded with number of years engaged in night work.

In a recently published French case-control study the risk for ER+ and PR+ breast cancer doubled and for risk for HER2+ cancers nearly tripled in women working nights shifts.


The first time I noticed this idea of measuring light at night was in a 2008 study by Kloog and colleagues at the University of Haifa.  They used NASA night-time satellite photos to estimate the LAN levels in 147 communities in Israel and found a strong positive association with breast cancer rates. They reporteda 73% higher breast cancer incidence in the highest LAN exposed communities compared to the lowest LAN exposed communities.  


Similar findings have been reported in studies done in all parts of the world.

Night-time does not affect all cancer types, it seems to raise risk of just breast and prostate cancer but not others.


Something as simple as closing shutters or drawing the shades at bed time may reduce breast cancer risk by 18%.   It is amazing just how much benefit simple changes might make.


Given the weight of the evidence we need to start treating light at night a carcinogenic environmental pollutant and strive to reduce exposure.


There is a fascinating website devoted to just 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.

Light at Night




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.;jsessionid=9741E2CCD287258B029B38E3C23E0997.f04t02


Stevens, R.G., Brainard, G.C., Blask, D.E., Lockley, S.W. ,Motta, M.E., 2014. Breast cancer and circadian disruption from electric lighting in the modern world. CA Cancer J. Clin. 2014. 64, 207–218.


Knutsson A, Alfredsson L, Karlsson B, Akerstedt T, Fransson EI, Westerholm P, Westerlund H. Breast cancer among shift workers: results of the WOLF longitudinal cohort study. Scand J Work Environ Health. 2013 Mar 1;39(2):170-7.


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.