In 2007, the International Agency for Research on Cancer, part of the World Health Organization (WHO), classified shift work resulting in circadian disruption as a “probable” carcinogen due to substantial evidence from both animal and human studies.
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Breast cancer is the most extensively researched type of cancer in relation to shift work. A 2012 report by Bonde et al. indicated that individuals working night shifts for 20 years or more face a statistically significant increase in breast cancer risk. However, the existing studies did not clarify whether shorter durations of night work pose similar risks. The authors also advised against night shifts for women with a history of breast cancer, citing strong evidence that melatonin secretion suppression—caused by exposure to light during nighttime work—accelerates tumor growth.
Research linking shift work to other cancers is less extensive. Among five studies reviewed, three indicated an increased risk for prostate cancer. Costa et al. identified potential mechanisms for these cancer risks, including night-time light exposure that lowers melatonin levels, disruptions in sleep and activity schedules, and diminished immune surveillance against cancer cells.