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Background: Modern electric lighting practices have transformed our society into one in which people work, sleep, and receive goods and services 24/7. As the light-dark cycle is the major stimulus for coordinating the circadian system, exposure to light at night (LAN) can lead to circadian disruption (CD) and potential adverse health effects.
Methods: The U.S. National Toxicology Program (NTP) conducted a cancer hazard evaluation of exposure to LAN that included a systematic review of epidemiology studies of female breast cancer. The human studies assessed LAN exposure in the sleeping area (i.e., “indoor light”) or from outdoor light, mostly by using satellite imaging methods (e.g., “environmental LAN”). We characterized all identified studies as low or moderate quality based on our bias and sensitivity evaluation.
Results and discussion: The 10 studies of indoor LAN provided inconsistent findings due to heterogeneity in methods used to measure LAN levels and the lack of specificity regarding LAN levels. Cohort or case-control studies of environmental LAN consistently found an increased risk of breast cancer among women in the highest category of overall LAN or blue-light LAN exposure. Whether these measurements provide a relevant direct measure of light or are surrogates for other activities enabled by light is not clear. Furthermore, whether low light levels observed in these studies are sufficient to disrupt circadian rhythms remains a question. NTP concluded the epidemiological database was inadequate to evaluate the relationship between exposure to LAN and female breast cancer.
Conclusions: Mechanistic and toxicological data studies provide strong evidence that certain lighting conditions (excessive LAN and insufficient daylight) can cause CD, biological effects characteristic of carcinogens, and promote breast or mammary tumor growth, thus providing moderate confidence of carcinogenicity in humans. Additional studies precisely measuring exposure to all sources of LAN and characterizing LAN intensities and spectral characteristics are needed.