A new study from Harvard has found greater risk of breast cancer in women who live in neighborhoods that have higher levels of outdoor light during the night.
The findings are based on the Nurses’ Health Study (NHS), which has for decades been advancing our understanding of risks to women’s health. For this study, epidemiologist Peter James and colleagues followed nurses in the NHS for breast cancer occurrence from 1989 to 2013. The home of each of 109,672 nurses was geocoded, and the average light level in the immediate neighborhood at night was estimated from satellite images taken by the Defense Meteorological Satellite Program. These estimates were updated over the 15-year follow-up period. By 2013, a total of 3,549 new cases of breast cancer had been diagnosed, about what’s expected among this number of women.
The study found a direct relationship between a woman’s neighborhood nighttime light level before diagnosis and her later risk of developing breast cancer: The higher the light level, the higher the risk.
These findings held even when taking into account many other factors that may also affect risk such as age, number of children, weight, use of hormone medications and a long list of additional potential confounders. Of importance if confirmed in more studies, the relationship was strongest in young women diagnosed before menopause.
The study is significant because it adds a strong piece of evidence to the growing body of studies supporting the idea that excessive electric light exposure at night increases a woman’s risk of breast cancer.
Too much evening light can delay the normal transition to nighttime physiology that should begin at dusk. An important part of this transition is a substantial rise in the hormone melatonin in the blood. Melatonin has been shown to have strong anti-cancer effects in lab rats. The shorter the wavelength of the light – that is, light that has more blue relative to other colors – the greater impact on lowering melatonin and delaying transition to nighttime physiology.
The Harvard study is one of the best-equipped studies ever conducted of women’s health to deal with this possibility because it has individual-level data on so many characteristics recorded for each subject, such as age, weight, smoking, body mass index, income and on and on.
If the results from Harvard are real, and too much light at night from any and all sources does increase a woman’s risk of breast cancer, then retrofitting street lighting all across America should be done in a way that does not further contribute to the problem. It would be best to select luminaires that are as dim as possible, and weak in the short wavelengths (e.g., blue), yet still consistent with accomplishing their intended purpose.
Electric light is one of the signature inventions, but its overuse has caused an obliteration of night in much of the modern world. The loss of night has consequences for all forms of life, including us.
And the mounting evidence for a connection to breast cancer is alarming.
From an article by Richard G. "Bugs" Stevens, University of Connecticut
Hmmm...something to think about. Maybe this is why I go around turning off lights and enjoy relaxing by candlelight.
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