A recently published evaluation by IARC (International Agency for Research on Cancer) classifies night shift work as "likely to cause cancer in humans".
A group of 27 scientists from 16 countries recently updated the assessment of cancer risk posed by night shift work on behalf of the IARC, as reported in the July issue of Lancet Oncology.1-3
The working group looked at night shift work activities taking place during the regular sleeping hours of the general population, including transmeridian air traffic. Such shifts are particularly common in healthcare, manufacturing, logistics, retail, and services. The disruption of circadian rhythms in normal human physiology is the most pronounced effect of night work.
In both humans and animals, there is solid evidence that these shifts alter melatonin levels and the expression of central circadian genes. There is strong mechanistic evidence from experimental systems on the effects of circadian gene alteration on processes such as immunosuppression, chronic inflammation and cell proliferation, which are key properties of carcinogens.
In 2007, the IARC had already classified "work shifts with circadian rhythm disturbance" as “below 2A” (probably carcinogenic to humans). A large number of new high-quality epidemiological studies (published after 2007) were used for the current updated evaluation. However, the panel noted considerable differences in the information available on night shifts, e.g. case-control studies provided more detailed information on exposure than cohort studies.
The largest number of studies investigated breast cancer, several prostate, and colorectal cancer, and a smaller number of studies were dedicated to other tumor entities.
The evaluation was based on extensive literature research, review of studies according to established inclusion criteria, and assessment of study quality, including standardized exposure assessment. The most meaningful work on human cancer received greater weight by methodological features such as sample size, possible selection effects, the accuracy of night shift recording, and control for potential interfering variables. The working group concluded that the limited available evidence nonetheless suggests an increased risk of breast, prostate and colorectal cancer from night work.
The current assessment is based on limited evidence on human cancer, sufficient evidence on animal cancer and strong mechanistic evidence from animal models. The classification "probably carcinogenic" based on this data indicates one thing above all: that there is a need for further clarification. It does not include a quantitative indication of how much the risks increase in night shifts (and which are the exact risks), and the definition of the classification "probable carcinogen" ultimately leaves open how strong the link is.
People in jobs with frequent night shifts may differ from other workers in a number of occupational, individual, environmental and lifestyle factors that may also affect cancer risk. The extent, nature, and regulations also vary in different places and across different employment sectors.
While this poses a methodological challenge, the question remains as to why a risk factor that affects so many people is still not better understood. About one in five workers worldwide are employed in night shifts.
Sources & References
1. McConway, K. Cancer and night shift work: what we still do not know and why. The Lancet Oncology 20, 1051–1052 (2019).
2. Ward, E. M. et al. Carcinogenicity of night shift work. The Lancet Oncology 20, 1058–1059 (2019).
3. Night Shift Work Classified as Probably Carcinogenic to Humans. Available at: https://www.esmo.org/Oncology-News/Night-Shift-Work-Classified-as-Probably-Carcinogenic-to-Humans.