In June 2019, a Working Group of 27 scientists from 16 countries met at the International Agency for Research on Cancer (IARC) in Lyon, France, to finalise their evaluation of the carcinogenicity of night shift work. The Working Group classified night shift work in Group 2A, “probably carcinogenic to humans”, based on limited evidence of cancer in humans, sufficient evidence of cancer in experimental animals, and strong mechanistic evidence in experimental animals. A summary of the evaluations is published in The Lancet Oncology on 4 July 2019. The full assessment will be published in volume 124 of the IARC Monographs.
The Working Group described that night shift work involves work, including transmeridian air travel, during the regular sleeping hours of the general population. Disruption of circadian rhythms of normal physiology is the most pronounced effect of night shift work.
Night shift is common in healthcare, manufacturing, transport, retail, and services sectors. Around 1 in 5 workers worldwide are engaged in night shift work. But it should be noted that definition, quality, and extent vary globally. Furthermore, regulation of night shift work differs across regions and employment sectors.
In 2007, the IARC classified shift work involving circadian disruption as “probably carcinogenic to humans” (group 2A), on the basis of sufficient evidence in experimental animals and limited evidence of breast cancer in humans.
In newly updated evaluation, the Working Group selected a term ‘night shift work’ in order to better describe the exposure circumstances and to reflect the main evidence base for the human cancer studies.
The re-evaluation was performed upon the large number of new, high-quality epidemiologic studies including additional cancer types. However, the Working Group noted a considerable variability in information on night shift work reported in these studies. For example, exposure information was more detailed in case-control studies than in cohort studies.
A number of occupational, individual, lifestyle, and environmental factors might mediate, confound, or moderate potential cancer risk in night shift workers.
The largest number of informative studies examined breast cancer, several examined prostate and colorectal cancer, while fewer were done on other cancers.
The Working Group concluded there was limited evidence that night shift work causes breast, prostate, and colorectal cancer. This evaluation was based on comprehensive searches of the literature, screening of the studies using established inclusion criteria, and evaluation of study quality, including a standardised review of exposure assessment. Greater weight was given to the most informative human cancer studies based on methodologic considerations, including study size, potential selection bias, night work assessment quality (most notably, potential for misclassification), and control for potential confounding factors.