Light at night ups cancer risk in big nurses’ health study

If you want to avoid breast cancer, you may want to consider avoiding nightime light and working the graveyard shift.

According to a new Harvard study, women living in areas with a lot of outdoor light at night appear to be at a higher risk of developing breast cancer than women living in less lit areas. The effect was even more pronounced among women working the night shift.

The data comes from a study by Harvard T.H. Chan School of Public Health published in Environmental Health Perspectives.

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

Other studies have hinted that light at night may lower levels of melatonin, which helps regulate our internal clocks or circadian rhythm. And that disruption, it’s been thought, can lead to higher risk for breast cancer. The current study looked at 110,000 women enrolled from 1989-2013 in the Nurses’ Health Study II.  Scientists tied information from satellite images of Earth taken at nighttime to home addresses for each study participant. They also looked at whether the women worked at night, considered participants’ health and socioeconomic factors.

After crunching the numbers, researchers found that women exposed to the most outdoor light at nighthad about 14% more chance of having breast cancer during the study period compared to those with the least exposure. The light-cancer link occurred only among women who had not yet reached menopause, and who smoked or once had.

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