A new study from Harvard’s T.H. Chan School of Public Health found women exposed to the highest levels of outdoor light at night – those in the top fifth – had an estimated 14% increased risk of breast cancer, compared with women in the bottom fifth of exposure.
In the study, researchers looked at health data from over 110,000 women and linked their addresses to satellite images of Earth taken at night. From that, they were able to see a spike in risk for those exposed to outdoor light at night.
The link was only found among premenopausal women – ie, anyone who has not yet hit menopause – and current or past smokers. The link was also stronger for women who worked night shifts.
So why is light linked to increased risk? Past studies have found that if you’re exposed to light at night, it can decrease your melatonin and mess with your body clock. Researchers believe that the decrease in melatonin may disturb estrogen regulation – which leads to increased breast cancer risk. Tree change, anyone?