- Study found direct relationship between neighborhood nighttime light level and woman's risk of breast cancer
- Too much evening light can delay the normal transition to nighttime physiology
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.