Postmenopausal women living in areas with high levels of outdoor light had an increased risk of developing breast cancer, according to results of an analysis of data collected as part of a large prospective cohort study that was published in the International Journal of Cancer.
Results of epidemiological studies have shown an increased risk of breast cancer in women who work night shifts over long-term periods compared with the general population. It has been proposed that decreased production of melatonin at night, as well a change in eating habits, resulting from disruption of circadian rhythm associated with increased exposure to light at night may at least partially underlie this finding. However, the results of studies evaluating the association between exposure to light at night and breast cancer risk have been mixed.
In this study, the effect of outdoor light at night on the breast cancer risk of 186,981 postmenopausal women was evaluated using data collected as part of the National Institutes of Health-American Association of Retired Persons (NIH-AARP) Diet and Health Study.
Incidence of breast cancer was determined using cancer registry data, and the level of outdoor light at night was evaluated using satellite imagery data linked to the geocoded address at baseline during 1996 of each individual included in the study. Demographic and lifestyle characteristics, including ethnicity, body mass index (BMI), smoking history, dietary intake and reproductive history, were collected as part of the NIH-AARP study, and information related to population density and median home value were gleaned from U.S. Census data according to baseline address.
At baseline, the median age of women included in this analysis was approximately 62 years. Women in the lowest quintile with respect to baseline level of light at night were more likely to be white-non Hispanic (95.4% vs 79.3%), married (54.6% vs 33.8%), have a history of using menopausal hormonal therapy (55.2% vs 47.5%), and to report 7 to 8 hours of sleep at night (61.7% vs 54.4%) compared with those in the highest quintile; however, they were less likely to be a current smoker (13.8% vs 15.5%), nulliparous (11.0% vs 18.3%), to have had their first child after age 30 years (4.4% vs 6.2%), and to have attended college (23.9% vs 29.6%).
Over approximately 16 years of follow-up, 12,318 cases of postmenopausal breast cancer were documented.
A key study finding was that those women in the highest quintile with respect to baseline light at night had a significantly higher risk of postmenopausal breast cancer (hazard ratio [HR], 1.10; 95% CI, 1.02-1.18; P =.002) compared with women in the lowest quintile for light at night.
In addition, when separate analyses were performed according to breast cancer estrogen receptor (ER) status, the HRs were 1.12 (95% CI, 1.02-1.24; P =.007) and 1.07 (95% CI, 0.85-1.34; P =.66) for those with ER-positive and ER-negative disease, respectively, for this comparison.
“The somewhat stronger associations with [ER-positive] breast cancer support an estrogen-specific mechanism of [light at night] and are consistent with the melatonin hypothesis, which postulates that [light at night] exerts its carcinogenic effects by suppressing the nocturnal release of melatonin, a hormone that inhibits both estrogen metabolism and ER-mediated signaling pathways,” the study authors noted.
Although stronger associations between light at night and breast cancer risk were observed among women who reported being a current smoker or drinking more than 1 alcoholic beverage per day, they did not reach statistical significance.
Some of the limitations of this study mentioned by the study authors included the lack of information related to certain lifestyle and environmental factors that may influence exposure to light at night, including type of window coverings, location of bedroom with respect to outdoor light sources, and work schedule.
In their concluding comments, the study authors remarked that “future studies should incorporate more refined [light at night] measurements at the personal level and include women from diverse racial/ethnic backgrounds and age groups.”
Xiao Q, James P, Breheny P. et al. Outdoor light at night and postmenopausal breast cancer risk in the NIH‐AARP diet and health study [published online June 2, 2020]. Int J Cancer. doi: 10.1002/ijc.33016