IARC Monographs on the Identification of Carcinogenic Hazards to Humans Volume 129
Summary
The International Agency for Research on Cancer (IARC) reviewed decades of human and animal studies on night-shift work and cancer risk. In 2019, it classified night-shift work as “probably carcinogenic to humans” (Group 2A). This conclusion was based on limited but suggestive evidence in humans (mainly breast, prostate, and colorectal cancer in long-term night workers) and sufficient evidence in animal models showing that circadian disruption can promote tumor development.
Why It Matters For Night Shift Workers and Night Owls
This classification doesn’t mean night work always causes cancer, but it does signal that chronic circadian disruption is a recognized biological stressor. For night-shift workers, the value lies in awareness: understanding that long-term exposure may carry risk helps support calls for better protections, smarter scheduling, and lifestyle strategies that reduce circadian disruption. It also explains why cancer prevention groups and health agencies continue to monitor shift work as a priority issue.
Notes
Full monograph PDF available from IARC.