Night Shift Work (IARC Monographs on the Identification of Carcinogenic Hazards to Humans, Volume 124)
Summary
The International Agency for Research on Cancer (IARC) reviewed epidemiologic studies, animal experiments, and mechanistic data to evaluate cancer risks from night-shift work. In humans, studies showed positive associations between long-term night work and breast, prostate, and colorectal cancers, though results were inconsistent and evidence was judged limited. In animals, altering the light–dark cycle produced sufficient evidence of carcinogenicity, with increased rates of liver, lung, and lymphoma tumors. Mechanistic studies provided strong support, showing that circadian disruption reduces melatonin, alters hormone regulation, and interferes with DNA repair. Based on this combined evidence, IARC classified night-shift work as “probably carcinogenic to humans” (Group 2A).
Why It Matters For Night Shift Workers and Night Owls
This doesn’t mean every night worker will develop cancer, but it shows that long-term disruption of the body’s natural day–night cycle can raise risks. The evaluation suggests that the duration, intensity, and scheduling of night shifts all matter. For workers and employers, strategies that protect circadian health — such as better shift design, exposure to bright light at the right times, and adequate recovery periods — may help reduce long-term health risks.
Notes
Open-access monograph PDF available from IARC.