Blue light and breast cancer

Original Source Here
By: Gary L. Morgan, OD

I want to start by thanking Alan Burt for writing my last blog post, reporting on the 2015 New York Blue Light Symposium.

As Alan pointed out, this event was an unintentionally well-kept secret from the optometric community. Alan happened across a blurb about the event just 1 week before it took place and worked hard to get there on short notice. I certainly plan on attending the event next year and will announce the date when known; as a profession, we should have a strong presence at this conference.

As Alan reported, the keynote address discussed the American Medical Association Policy Statement regarding nighttime lighting as harmful to human health and potential carcinogenic effects. An excerpt from this states: “In various laboratory models of cancer, melatonin serves as a circulating anticancer signal and suppresses tumor growth” and “epidemiological studies support the hypothesis that nighttime lighting and/or repetitive disruption of circadian rhythms increases cancer risk; most attention in this arena has been devoted to breast cancer.”

Delving a little deeper, I believe the “nighttime lighting” component should be clarified.

Specifically, numerous studies state that melatonin suppression is greatest between wavelengths of 459 nm to 484 nm, and that 460 nm light is twice as strong for suppressing melatonin as either 420 nm or 555 nm light. Because blue light is defined as 400 nm to 500 nm, perhaps the statement should read “blue light at night” instead of nighttime lighting.   

I recently came across a website for Brian Lawenda, MD, a radiation/integrative oncologist. In it he highlights a study published last year that demonstrated that light exposure at night increases breast cancer growth and inhibits tamoxifen

In one part of the study, rats were implanted with breast cancer cells and divided into two groups, one kept in complete darkness at night and the other being exposed to dim light at night, approximately what might come under a bedroom door from a lit hallway. The group kept in complete darkness at night had notably slower development of breast cancer and tumor growth, suggesting that complete darkness at night suppresses cancer.

In a second part of the study, two groups of rats with breast cancer were started on tamoxifen. One group was kept in complete darkness at night and the other group was exposed to dim light at night. The group kept in complete darkness at night, which maintained normal nighttime melatonin levels, had a dramatic reduction in tumor size and growth. For the group exposed to dim light at night, the anticancer effect of tamoxifen was negated, suggesting normal plasma melatonin levels are necessary for tamoxifen to be effective.  

In a third part of the study, to determine if the anticancer effect of tamoxifen really was related to melatonin levels, rats implanted with breast cancer cells were given supplemental melatonin without tamoxifen and exposed to dim light at night. Results showed that melatonin by itself significantly delayed tumor formation and growth for these rats exposed to dim light at night. When rats with breast cancer tumors were kept in dim light at night, supplemented with melatonin and were given tamoxifen, the tamoxifen caused significant tumor regression. So, essentially, melatonin supplementation allowed for the normal action of tamoxifen for the rats exposed to dim light at night.  

The authors concluded that high melatonin levels at night slowed the growth rate of breast cancer cells, making them vulnerable to tamoxifen. However, light at night suppressing melatonin allows breast cancer cells to continue to grow rapidly, and tamoxifen loses its efficacy.

Dr. Lawenda concludes in his review that we should heed the results of this study now. Block light at night in your bedroom and get 6 to 7 hours of sleep, as cancer likes light exposure at night. He also makes an interesting statement at the end of his article: “If there is no way for you to avoid being exposed to light at night, blue light-blocking filtered glasses and goggles can prevent melatonin suppression.”

So, while the study authors did not specifically mention wavelength of light for their dim light at night conditions, Dr. Lawenda connects the dots, which clearly puts the ball in our court. As an industry, we need to continue to work on solutions for patients regarding blue light exposure at night. This seems to have optometry written all over it, wouldn’t you agree?


AMA House of Delegates 2012 Annual Meeting: Council on Science and Public Health, Report 4. Light pollution: Adverse health effects of nighttime lighting.

Brainard G, et al. J Biol Rhythms. 2008;23(5): 379-386.

Dauchy RT, et al. Cancer Res. 2014;74(15):4099-4110. doi: 10.1158/0008-5472.

Lawenda BD. Light exposure at night increases breast cancer growth and inhibits tamoxifen. Integrative Oncology Essentials. Posted July 25, 2014. Accessed August 18, 2015.

Lockley SW, et al. J Clin Endocrinol Metab. 2003;88(9):4502-4505.

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