High exposure to light at night (LAN) was associated with an increased risk for thyroid cancer, particularly among women, according to results of a cohort study published in Cancer.

“To our knowledge, this is the first study to evaluate the hypothesis that greater exposure to LAN is associated with a higher risk for thyroid cancer,” the study authors wrote.

Data from recent studies have suggested that disruption of circadian rhythm may be a risk factor for thyroid cancer. LAN inhibits the nighttime secretion of melatonin, potentially causing circadian disruption. The study authors sought to determine whether exposure to LAN is associated with risk for thyroid cancer.

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The study included 464,371 adults from the National Institutes of Health–American Association of Retired Persons (NIH-AARP) Diet and Health Study aged 50 to 71 years at baseline from 1995 to 1996. The investigators identified individuals who developed thyroid cancer by linking to state cancer registry databases. They analyzed data from the US Defense Meteorological Satellite Program (DMSP), a database that includes satellite imagery of outdoor LAN, to document LAN exposure.

“We did not have information on indoor LAN levels or important factors that may influence the LAN exposure, such as nighttime activities, sleep schedules, window treatments, and illumination at home,” the study authors said.

LAN was found to be significantly associated with an increased risk of developing thyroid cancer after multivariate adjustment, including median home value, poverty rate, and population density. Compared with the lowest quartile of LAN, the risk for thyroid cancer increased by an HR of 1.12 (95% CI, 1.06-1.19) per quartile, with an HR of 1.27 (95% CI, 1.01-1.61) in the third, 1.51 (95% CI, 1.19-1.91) in the fourth, and 1.55 (95% CI, 1.18-2.02) in the fifth quartile (trend P =.0002).

The association was also apparent when the investigators evaluated the risk for papillary thyroid cancer, with an HR of 1.61 (95% CI, 1.18-2.21; trend P =.001) in the highest quartile compared with the lowest quartile. There was no association for follicular or medullary subtypes.

When stratified by gender, women had a significantly increased risk of developing thyroid cancer, with an HR of 1.81 (95% CI, 1.26-2.60; trend P =.001) at the fifth quartile compared with the first. There was no significant association at the fifth quartile among men (HR, 1.29; 95% CI, 0.86-1.94).

Among women, the risk of developing thyroid cancer was primarily driven by localized disease (HR, 1.95; 95% CI, 1.27-2.99; trend P =.01) rather than regional/distant disease (HR, 1.39; 95% CI, 0.61-3.17; trend P =.10). By contrast, LAN was associated with an increased risk for regional/distant thyroid cancer among men (HR, 2.63; 95% CI, 1.24-5.58; trend P =.01) but not localized disease (HR, 1.05; 95% CI, 0.58-1.89; trend P =.38).

Several subgroup associations were identified between baseline LAN and the incidence of all thyroid cancer, including younger age, White race, some level of college education, and a poverty rate of less than 6.3% the census tract.

We found that higher LAN levels were associated with higher thyroid cancer incidence in middle-aged to older adults in the United States,” the study authors concluded, adding that additional studies are needed to confirm their findings and to identify the underlying mechanism driving the association.


Zhang D, Jones RR, James P, Kitahara CM, Xiao Q. Associations between artificial light at night and risk for thyroid cancer: a large US cohort study. Cancer. Published online February 8, 2021. doi:10.1002/cncr.33392