Breast Cancer & Circadian Rhythm Disruption from Electric Lighting

woman sleeping serenely

What You Need to Know: With industrialization comes electricity to light the night, both within the home and outside of it. A convergence of research in cells, rodents, and humans suggests that the health consequences of circadian disruption may be substantial. Evidence is accumulating to suggest that exposure to light at night may present a risk for breast cancer. It is not unwise to keep your night sleep time as dark as possible.

Background: Breast cancer is the leading cause of cancer death among women worldwide. Risk is highest in economically developed countries, and is rising rapidly in places that have historically had a low risk of the disease. Until the 1980s the researchers believed that this increase might be primarily related to diet. Yet, the evidence linking fat content in the adult diet to breast cancer (and showing decreases with fruit and vegetable consumption) is rather weak. In fact, other than alcohol, overall diet composition has not been strongly associated with breast cancer risk. Body mass has been linked, however. In fact, less than half of the risk in high-risk societies can be attributed to known risk factors. While recent evidence shows that physical activity can lower risk, a decrease in such activity is not likely accountable for the additional risk found in industrialized countries. So, besides obesity, less physical activity, and the use of hormones, could increased exposure to light during the dark hours (which can disrupt the “sleep hormone” melatonin be a culprit?

breast cancer tumor

The Evidence Linking Circadian Rhythm Disruption and Breast Cancer: The first suggestion that light at night might explain a portion of the breast cancer pandemic was made in 1987. The hypothesis was based on the idea that exposure to light at night would result in melatonin suppression, which in turn would increase breast cancer risk as described in the previous section. Since 1987, a series of predictions of this theory have been tested. Let’s turn to some of the evidence:

  • Shift work: Women who work nights (shift work) are at higher risk of breast cancer. The International Agency for Research on Cancer concluded that “shift work that includes circadian rhythm disruption is probably carcinogenic to humans.” The American Medical Association then broadened the topic in 2012 on the health habits of light at night in general. While subsequent studies have had mixed results, a meta-analysis (including a study of high-quality studies) linked night work to an increased risk (by a factor of 1.4).
  • Blind women: Four studies have suggested that blind women may be at a lower risk of breast cancer than sighted women.
  • Ecological analyses: Kloog analyzed cancers (lung, colon, larynx, and liver) in 164 countries. Breast cancer was significantly associated with nighttime illumination, and it was estimated that the risk was increased by a factor of 1.3 to 1.5, compared with the lowest lighted countries. The investigators controlled for fertility rate, per capita income, percent of urban population, and electricity consumption.
  • Genetics (circadian gene polymorphisms): Researchers first linked a circadian gene polymorphism to breast cancer risk in 2005.
  • Differences in methylation of circadian genes have been found when comparing day shift and night shift workers, offering a possible mechanism for an increased risk among night workers.

My Take: It is clear that electric lighting, including indoor evening light levels, has strong effects on our circadian rhythms. Recent studies show that the lighting used in at the typical home (in the industrialized world) is enough to delay melatonin onset and blunt its night peak. So here is what I would like you to do:

  • Limit your night exposure to light, whether from computer screens (the emitted light in the blue spectrum changes melatonin levels) or to night lights.
  • Sleep in a dark room, if possible (no light from the clock next to the bed, or seeping into your room from street lights, for example). Nocturnal light exposure and circadian disruption may be particularly important for children, and even exposure to light as a pregnant woman may affect fetal exposure to hormone levels in utero.

So many questions remain: Might circadian rhythm disruption affect your response to chemotherapy? Could it affect the natural history of breast cancer and affect the risk or pace of progression? Can we control our lighting in a more sophisticated fashion that might not be so disruptive? I’m Dr. Michael Hunter, and I thank you for reading this very long blog.

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.

Available now: Understand Colon Cancer in 60 Minutes; Understand Brain Glioma in 60 Minutes. Both can be found at the Apple Ibooks store. Coming Soon for iPad: Understand Breast Cancer in 60 Minutes; Understand Colon Cancer in 60 Minute; Understand Colon Cancer in 60 Minutes; Understand Brain Glioma in 60 Minutes. Thank you.

Reference: RG Stevens et al. Breast Cnacer and Cricadian Disruption from Electric Lighting in the Modern World. CA Cancer J Clin 2014;64: 207-218.


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

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