Night shift work has been associated with breast, prostate, and colorectal cancers. However, conclusions on its carcinogenicity are inconsistent between organizations that have evaluated the research. For example, the International Agency for Research on Cancer has classified night shift work as probably carcinogenic to humans (Group 2A) , while the National Toxicology Program has proposed that it be listed as known to be a human carcinogen.

Given that approximately 1.8 million Canadians work night shifts between midnight and 5 am, the potential impact of night shift work on cancer may be large. Industry groups with the greatest numbers of people working night shifts are the trades (n=396,000; 15% of industry), health care and social assistance (n=318,000; 18% of industry), and manufacturing (n=261,000; 17% of industry). According to the Burden of Occupational Cancer study, night shift work could be responsible for 470 to 1,200 breast cancer cases each year in Canada. In response to the ongoing uncertainty in this research, our team conducted a review of the literature to identify the main data gaps and challenges that are faced when assessing the carcinogenicity of night shift work. The results fall under three main areas:

  1. Exposure assessment: The effects of night shift work are particularly difficult to study because night shift work is an exposure circumstance and not a defined substance. The specific circumstance that may lead to increased cancer risk is still not clear. However, studies continue to collect inadequate exposure information to ascertain those scenarios under which cancer risk may be increased. In addition, inconsistent definitions of night shift work are used, making it difficult to compare and combine results across studies.
  2. Epidemiology: Many studies fail to capture the population at the highest risk of cancer. For example, research suggests that the effect of night shift work on breast cancer may wane over time; however, many studies include workers who were exposed many years ago. By selecting workers who may not represent those at most risk, study results may be underestimating the cancer risk. Furthermore, studies are faced with low case numbers and short follow-up times, which make it difficult to detect outcomes due to low statistical power.
  3. Mechanism of action: Multiple mechanisms of action (i.e. pathways through which night shift work could contribute to cancer outcomes) have been proposed. Due to their overlapping and interrelated nature, teasing apart the potential contributions of each mechanism to the formation of cancer is difficult. However, further investigation into the mechanism(s) of action should be pursued in the assessment of shift work’s carcinogenicity if the epidemiologic and exposure assessment challenges cannot be adequately addressed.

For more information, the full report is available here.

The International Agency for Research on Cancer’s full monograph on the carcinogenicity of night shift work was released in June 2020 and is available here.

To review the National Toxicology Program’s draft monograph, click here.

 A range of estimates is presented to account for uncertainty in the magnitude of cancer risk associated with night shift work exposure.

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