Nighttime Light Exposure is Associated with Breast Cancer

Nighttime Light Exposure is Associated with Breast Cancer

Take-Home Message:

Nighttime light exposure appears to play a role in the development of breast cancer.


Outdoor Light at Night and Breast Cancer Incidence in the Nurses’ Health Study II

Authors: James, P. et. al.

Source: Environmental Health Perspectives

There is abundant research about nighttime light exposure and breast cancer development. Recent work has shown that women who work night shifts are more likely to develop breast cancer. Studies show that light at night (LAN) decreases melatonin secretion, which disrupts the circadian system. This may lead to a disturbance of estrogen regulation and subsequent breast cancer.  

The investigators used this information to evaluate the development of breast cancer in women in the United States. The Nurses’ Health Study II is an ongoing project that follows over 110,000 women who were ages 25-42 in 1989. Every two years, these women complete questionnaires to provide information about their health and lifestyles.

Follow up for this study was 22 years. Reported cases of breast cancer were confirmed by medical records, cancer registries, death records or direct contact with study participants or their families. Data on outdoor LAN levels for the US was provided by the US Defense Meteorological Satellite Program, which maintains a database about annual light originating from the Earth’s surface across the globe. Distribution of the breast cancer cases was compared to the distribution of LAN exposure in the places where the women lived.

There were over 3,500 breast cancer cases during the study period. The greatest risk of breast cancer was in regions with the greatest LAN. This association was specific to women diagnosed with premenopausal breast cancer and in former and current smokers. There was no association between LAN and breast cancer based upon body mass index, race, income or urban/nonurban status.

This interesting study provides evidence of the role of environment in causing breast cancer and suggests that women who live in areas with high levels of outdoor LAN may be at increased risk for the disease. This provides guidance for further research into the association between outdoor LAN and breast cancer.