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Outdoor Light at Night Increases Breast Cancer Risk

Outdoor Light at Night Increases Breast Cancer Risk
(Copyright DPC)

By    |   Thursday, 17 August 2017 11:47 AM

A long-term study from Harvard T.H. Chan School of Public Health found that women who live in areas with high levels of outdoor light at night have a higher risk of breast cancer than those who live in darker areas.

The researchers examined data from nearly 110,000 women enrolled in the Nurses' Health Study II from 1989-2013. They then connected data from satellite images of Earth taken at nighttime to residential addresses of women in the study. They also considered the influence of night shift work as well as information on their health and socioeconomic status.

"In our modern industrialized society, artificial lighting is nearly ubiquitous," said lead author Peter James. "Our results suggest that this widespread exposure to outdoor lights during nighttime hours could represent a novel risk factor for breast cancer."

Previous studies have suggested that exposure to light at night may lead to decreased levels of the hormone melatonin. Lower levels disrupt circadian rhythms, the internal clock that governs sleep and wake cycles, which leads to increased breast cancer risk.

The Harvard study found that women exposed to the highest levels of outdoor light at night increased their risk of breast cancer by 14 percent compared to women who received the least exposure.

The association between outdoor light at night and breast cancer was found only among women who were premenopausal and those who were current or past smokers. In addition, the link was stronger among women who worked night shifts, suggesting that exposure to light at night and night shift work contribute jointly to breast cancer risk.

The study is published in Environmental Health Perspectives.

A 2015 study published in the journal Biology, found that women who work outside the normal work hours of 9 to 5 have an increased risk of breast cancer.

Dutch researchers subjected mice to disturbances in their circadian rhythm, and found that alternating light-dark cycles on a weekly basis accelerates the development of breast cancer.

Female mice with a genetic predisposition to breast cancer, similar to humans who carry a BRCA mutation, were studied. These mice usually develop breast cancer at about 50 weeks, but they developed the disease eight weeks earlier when subjected to circadian rhythm disturbances. In addition, they also gained 20 percent more weight than mice who kept normal schedules.

"The conclusion is that chronic changes in light schedules are a driving factor for breast cancer development, weight gain, and other metabolic problems," said senior study author Bert van der Horst of Erasmus University Medical Center.

In addition, a British study of 113,000 women, which measured body fat by body mass index (BMI), waist circumference, and waist-to-hip ratio, found that the more light they were exposed to during sleeping hours, the greater their odds of being fat. Obesity is a known risk factor for breast cancer.

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A long-term study from Harvard T.H. Chan School of Public Health found that women who live in areas with high levels of outdoor light at night have a higher risk of breast cancer than those who live in darker areas.The researchers examined data from nearly 110,000 women...
outdoor, light, night, breast, cancer, risk
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2017-47-17
Thursday, 17 August 2017 11:47 AM
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