Reference summary — not medical advice or a personal risk estimate.
Night shift work
Quick answer
Classified as Group 2A — probably carcinogenic, based on limited evidence in humans, alongside stronger evidence in animals. Lifestyle. Regular work shifts during the biological night that disrupt circadian rhythms — common in healthcare, transport, security, and manufacturing.
Summary
Shift work that disrupts circadian rhythms is possibly associated with breast and other cancers.
Detail
IARC classifies night shift work as Group 2A with limited evidence for breast, prostate and colorectal cancer. The proposed mechanism involves circadian disruption and reduced melatonin secretion.
What's not established
This classification reflects limited evidence in humans, usually together with sufficient evidence in animals. It does not establish how strong the human evidence would look on its own, a safe exposure threshold, or your personal risk from a given level of exposure.
Regional relevance
global
Documented in these products
Presence in a product category doesn't imply the product causes cancer. Click each to see the source and context.
Evidence sources
Every claim on this page traces back to one of these. Click through to read the primary document yourself.
Primary sources
IARC
IARC Monographs — Classifications by Cancer Site, Vols 1–140
Supporting sources
ACS
ACS — Known and Probable Human Carcinogens
NTP
NTP — Report on Carcinogens (US National Toxicology Program)
Last reviewed: October 2026
Official reference list
A government list some readers use alongside the primary evidence. Meyron is not affiliated with it. A chemical being listed does not by itself describe a risk from any particular product.
- California Prop 65 list
State of California's list of chemicals known to cause cancer or reproductive harm.