National Toxicology Program (NTP). 2021. NTP Cancer Hazard Assessment Report on Night Shift Work and Light at Night. Research Triangle Park, NC: National Toxicology Program. https://doi.org/10.22427/NTP-CHR-NSWLAN
Report Date: April 2021
Modern electric lighting practices have facilitated a society in which people may work, sleep, and receive goods and services at any time of the day. People are exposed to ill-timed, unnatural, electric light (such as light at night, or “LAN”) through lifestyle choices, necessity, the locations of their residences, and employment during the night shift. As light is the critical regulator for circadian rhythms, exposure to LAN can cause circadian disruption, which can be associated with potential adverse health effects such as cancer. Night shift work includes exposure to electric LAN, sleep disturbances, or changes in meal timing, as well as other potential factors (e.g., social stressors, lifestyle behaviors, decreased exposure to sunlight, and lower vitamin D levels). Most, but not all, of these factors can lead to circadian disruption.
NTP final cancer hazard conclusions
There is high confidence for a causal relationship between human cancer (breast and less so prostate) and persistent night shift work — i.e., frequent and long-term night shift work, especially beginning in early adulthood — that causes circadian disruption. This conclusion is based on sufficient evidence of carcinogenicity from the collective body of evidence from cancer epidemiological studies and mechanistic studies in humans and in experimental animals. The strongest evidence is for breast cancer.
There is moderate confidence for a causal relationship between human cancer and certain lighting conditions — i.e., excessive LAN exposure combined with insufficient daylight exposure — that cause circadian disruption. This conclusion is based on strong evidence that LAN acts through mechanisms that are likely to cause cancer in humans.
The full report pdf is available at https://doi.org/10.22427/NTP-CHR-NSWLAN