The headlines read like something out of The Onion, but both the U.S. National Toxicology Program (NTP) and the International Agency for Research on Cancer (IARC) are undertaking reviews to assess whether working at night poses a cancer risk. Scientifically speaking, the agencies are examining whether practices that disrupt circadian rhythms, such as “light at night” (LAN), for which night shift work is viewed as a proxy, are associated with an increased cancer risk.

While neither NTP nor IARC have direct regulatory authority, both agencies often are cited as “authoritative bodies” for identifying carcinogens under regulations implemented by EPA, OSHA, and other agencies, as well as California Proposition 65.

In fact, this is IARC’s second look at the issue in the last decade: in 2010, the Lyon, France-based organization classified “shift work that involves circadian disruption” as probably carcinogenic to humans (Group 2A). That conclusion was based on (1) “limited evidence in humans for the carcinogenicity of shift work that involves night work,” and (2) “sufficient evidence in experimental animals for the carcinogenicity of light during daily dark period (biological night).” On August 14, IARC announced that it would once again examine the association between cancer and circadian disruptions caused by shift work, and has requested that relevant studies and other information be submitted by May 6, 2019.

Meanwhile, in May, NTP released a protocol outlining its approach to preparing the cancer hazard evaluation for a draft Report on Carcinogens (RoC) monograph on “Night Shift Work and Light at Night.” The aim is to assess “whether scenarios associated with exposure to modern electrical light practices that lead to circadian disruption, including light at night (LAN), shift work at night, and transmeridian travel, are associated with cancer risk.” Following a 2012 nomination for NTP to consider listing “shift work involving LAN” in the RoC, NTP convened an expert workshop in 2016 which recommended examining the “health consequences of electric lighting practices in the modern world.” NTP explains in the protocol document that

The rationale for this recommendation was that electric light acts as both an effector (based on direct effects on circadian disruption and melatonin suppression, and animal models and human studies of light pollution and indoor light), and as an enabler, allowing what were once daytime activities to be conducted 24/7. And thus, electric light as both an effector and an enabler of additional activities or behaviors (e.g., shift work), may lead to circadian disruption.

Key questions to be addressed in the monograph include:

• Do a significant number of people residing in the United States work night shifts? • Are a significant number of people residing in the United States exposed to LAN? • Should night shift work be listed in the RoC? If so, how should it be defined? • Can we define the underlying exposures related to circadian disruption? • Should LAN be listed in the RoC? If so, how should it be defined?

The NTP process offers multiple opportunities for public comment prior to reaching a final listing recommendation — and one can imagine that there are numerous potential confounding factors that will need to be addressed in order to establish a meaningful causal connection between night shift work/LAN and cancer risk. The ultimate listing determination will be made by the Secretary of the U.S. Department of Health and Human Services.