The short answer
IARC classifies night shift work as probably carcinogenic to humans (Group 2A), based on limited human evidence covering cancers of the breast, prostate, colon, and rectum, plus sufficient evidence in experimental animals. A decade of new studies left that classification unchanged, and IARC did not estimate how large the risk is.
Night shift work is classified as a probable human carcinogen (IARC Group 2A).
People are mainly exposed by long-term night or rotating shift work that disrupts the body clock.
The human evidence, graded limited, covers positive associations with cancers of the breast, prostate, colon, and rectum.
A carcinogen classification describes hazard — whether something can cause cancer — not your personal risk at a given exposure.
Choose how you want to read this
The whole article, with every detail and caution.
This page is for people who work nights or rotating shifts, and their families. It explains what IARC decided about night shift work and cancer, and what that label does and does not mean.
What exactly got evaluated
In June 2019, a Working Group of experts met in Lyon, France, for eight days. Their assignment was odd for the IARC Monographs programme. Most entries in that programme are chemicals. This one was a work schedule.
The agent arrived at the meeting under a clumsy name: "shift work that involves circadian disruption." The Working Group renamed it night shift work. The new name better matched the real exposure, and the studies that actually exist.
Their definition is precise and worth quoting. It is work during the regular sleeping hours of the general population. Two things follow from that wording. First, the exposure is defined by the clash with that sleep window, not by the job title. Second, the definition takes in transmeridian air travel. That means flying fast across many time zones, as cockpit and cabin crew do.
The 2019 verdict, and the one it replaced
The conclusion: night shift work is probably carcinogenic to humans, Group 2A.
That is not a change. IARC's earlier evaluation, Volume 98 in 2007, reached the same classification for shift work involving circadian disruption. So why meet again? IARC says the re-evaluation was motivated by the large number of new, high-quality epidemiological studies, including additional cancer sites. The new assessment is Volume 124.
Group 2A has a technical definition, and it explains the whole document. The category is used in one situation. There is limited evidence of cancer in humans, plus sufficient evidence in experimental animals. "Limited" is a term of art here. It means a positive link between exposure and cancer has been seen. But chance, bias, and confounding could not be ruled out as explanations.
Four cancer sites, and why the evidence stayed "limited"
The Working Group considered several kinds of data:
- Where and how people are exposed to night shift work.
- Studies of cancer in exposed people.
- Studies of cancer in experimental animals.
- Studies of the mechanisms by which the exposure might act.
IARC found limited evidence of cancer in humans for breast, prostate, colon, and rectum.
The evidence is graded limited because chance, bias and confounding could not be ruled out as explanations for the positive links.
Studies measured shift work in very different ways. Some counted any night work. Others counted years of rotating shifts, or consecutive nights. Some large studies found associations, and others found none. When the exposure itself is defined differently in every study, pooling the results becomes unreliable.
That is an honest description of a real state of knowledge. It is neither reassurance nor alarm.
What the animal and mechanism work adds
The evidence in laboratory animals is graded sufficient, the stronger term. Animals exposed to altered light-dark schedules developed cancer at increased rates.
The working group found strong mechanistic evidence. The effects were consistent with immunosuppression (a weakened immune system), chronic inflammation and cell proliferation (cells multiplying).
IARC also rated the mechanistic evidence in experimental systems as strong. In its summary, IARC says disruption of normal circadian rhythms is the most marked effect of night shift work. Those are the daily cycles that govern normal body function.
How many people this covers
IARC says night shift work occurs in health care, manufacturing, transport, retail and service sectors. It is how production and services continue 24 hours a day, seven days a week. That is why this exposure will be managed rather than eliminated.
What a Group 2A label does not tell you
IARC classifies hazards, not risks. A hazard is the ability to cause cancer under some circumstances. Risk is the chance that cancer will actually occur at a given level of exposure. The Working Group did not try to estimate risk at different amounts of night shift work. That would take a separate risk assessment, which is not what the Monographs do.
Our page on hazard versus risk works through that distinction, and how the IARC monographs work covers how the categories are assigned.
What this means in practice
For someone who has worked nights for years, the useful conclusions from Volume 124 are narrow.
IARC issued no recommendation that anyone should stop working nights. IARC does not make recommendations about individual actions. Its advice to concerned workers is that they "may consult your physician for advice."
The four cancer sites named are breast, prostate, colon and rectum. Ask your care team which screening fits your age and history, and mention your work schedule. A night roster is a common reason people fall behind on appointments. See breast cancer and colorectal cancer for what those pathways involve.
IARC did not estimate how large the risk is. A decade of new research left the classification where it was in 2007. More studies did not settle the question.
Sources
Common questions
Does night shift work cause cancer?
Probably. Night shift work is classified as a probable human carcinogen: the evidence in people is limited, but animal and laboratory studies support a link. "Probable" means suspected on solid grounds, not proven.
How are people exposed to night shift work?
Most exposure happens by long-term night or rotating shift work that disrupts the body clock.
Which cancers are linked to night shift work?
IARC's Working Group found positive associations with cancers of the breast, prostate, colon, and rectum. It graded that human evidence as limited, meaning chance, bias and confounding could not be ruled out. IARC classifies hazard, not the size of anyone's risk.
What should I do if I work nights?
IARC does not give advice to individuals. Ask your care team which screening fits your age and history, and tell them about your work schedule.
Does a carcinogen label mean I will get cancer?
No. A classification is about hazard — whether night shift work can cause cancer under some conditions — not a prediction that any one exposed person will develop cancer. IARC did not estimate risk at different amounts of night work.
Should I quit my night job?
IARC made no such recommendation. It does not make recommendations about individual actions, and it says people who are concerned may consult their physician for advice. The human studies are limited enough that no one can tell you how much risk you carry.
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Last updated: 2026-10-16Next planned review: 2028-07-05
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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