The researchers compared satellite imagery data — to estimate levels of nighttime light at each participant’s address — against data from state cancer registries through 2011.
They found 856 cases of thyroid cancer among the more than 464,000 participants, who were all followed an average of about 13 years. Those cases included 384 men and 472 women.
The researchers reported that people who lived in areas in the highest 20% category of nighttime light had a 55% higher risk of developing thyroid cancer, compared to those who lived in the lowest 20% bracket. The risk was primarily seen for the most common form of thyroid cancer, called papillary thyroid cancer.
Researchers also found the association was stronger in women than in men. They found that for women, the association was stronger for localized cancers that had no sign of spread to other parts of the body.
For men, the association was stronger for more advanced stages of cancer. The association was similar for different tumor sizes and participant demographics, including sociodemographic characteristics and body mass index (weight).
So how could light at night spur a thyroid cancer?
According to Xiao’s group, nighttime light suppresses natural melatonin, a modulator of estrogen activity. Too little melatonin activity might help depress the body’s ability to fight off tumors, they said. Light at night can also disrupt the body’s circadian rhythm, which is also a cancer risk factor, the researchers said.
Dr. Shuchie Jaggi is attending physician in endocrinology, diabetes and metabolism at Northwell Health in Great Neck, N.Y. Reading over the findings, she said it’s possible that nighttime light might affect the thyroid through its effect on hormones, and “certainly the large sample size of this study is a strength making these associations more statistically significant.”
Jaggi noted that the study had one shortcoming: Because it relied on satellite imagery, it could only examine levels of outdoor lighting at night, not whether folks kept their indoor lights on as well.
For his part, Har-El said that this is only “the first study to look into this aspect of advanced technology affecting our lifestyle, and it is only one study,” therefore, much more research is needed.