The short answer
IARC classifies night shift work as Group 2A, probably carcinogenic to humans, based on limited human evidence, sufficient animal evidence and strong mechanistic evidence. Large human studies have disagreed with each other. IARC makes no recommendation that anyone stop night work; it suggests concerned workers talk with their physician.
IARC classified night shift work as Group 2A, probably carcinogenic to humans, in its 2019 evaluation (Monographs Volume 124).
The human evidence was judged limited. Positive associations were seen with cancers of the breast, prostate, colon and rectum, but bias could not be ruled out with confidence.
Evidence in experimental animals exposed to altered light-dark schedules was judged sufficient, and mechanistic evidence was judged strong, showing effects consistent with immunosuppression, chronic inflammation and cell proliferation.
Human studies have disagreed with each other. Some large studies found associations and others did not, partly because they measured shift work in very different ways.
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The whole article, with every detail and caution.
This page is for people who work nights and have heard that shift work may cause cancer. It explains what the evidence shows and what steps are worth taking.
What this classification does and does not tell you
If you work nights, the useful thing to know first is what IARC actually said. The coverage tends to overshoot it.
In 2019 an IARC (International Agency for Research on Cancer) working group evaluated night shift work. It defined this as work during the regular sleeping hours of the general population. It classified night shift work in Group 2A, probably carcinogenic to humans. Group 2A is a statement about how confident scientists are that something can cause cancer. It is not a measurement of how much your own risk rises. And in this case nobody can give you that number reliably.
IARC issued no recommendation that anyone should stop working nights. Its guidance to concerned workers was to talk with their physician.
Why the evidence is mixed
The classification rests on three separate streams of evidence, and they did not agree.
Human studies: limited evidence. Positive associations were observed between night shift work and cancers of the breast, prostate, colon and rectum. But the working group could not rule out bias with confidence, and this is the honest sticking point. 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.
Animal studies: sufficient evidence. Animals exposed to altered light-dark schedules developed cancer at increased rates. This was judged sufficient evidence of carcinogenicity in experimental animals.
Mechanism: strong evidence. The working group found strong mechanistic evidence. The effects were consistent with immunosuppression (a weakened immune system), chronic inflammation and cell proliferation (cells multiplying).
Group 2A means "probably," not "known." Presenting it as a proven cause overstates it. Dismissing it as a scare understates it.
Questions worth raising
IARC does not make recommendations about individual actions. Its advice to concerned workers is that they "may consult your physician for advice." These are reasonable things to raise at that visit:
- Screening. Ask which cancer screenings are due for you, and whether early-morning, weekend or at-home options exist if clinic hours collide with your sleep.
- Sleep. If your sleep is poor on this schedule, ask what might help. That is a conversation about sleep, not about cancer.
- Other risks. Smoking, alcohol, physical activity and weight have well-established links to cancer. Ask your clinician which of these is worth working on for you.
What this label does not mean
This page found no approved source showing that melatonin or any other supplement lowers cancer risk in shift workers. Talk with your clinician before starting any supplement.
The Group 2A label does not tell you how much any one person's risk changes, and IARC made no recommendation that anyone stop working nights.
And do not read a past diagnosis backwards into your work history. It is not something you brought on yourself by taking the shifts that were available.
Sources
Common questions
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.
Why is it 'probably' rather than a clear yes or no?
Because the three streams of evidence point different ways. Animal studies with disrupted light-dark cycles gave sufficient evidence, and the biological mechanism is strong. But the human studies used inconsistent definitions of what counts as night shift work, over what number of years, and at what intensity, which makes them hard to pool. Some large studies found raised risk and others did not.
Which cancers were involved?
IARC noted positive associations with cancers of the breast, prostate, colon and rectum. IARC says these were the cancer sites with the most information. For all of them, bias could not be ruled out with reasonable confidence.
Does melatonin help?
This page found no approved source showing that melatonin supplements lower cancer risk in shift workers. Whether melatonin is useful for your sleep is a separate question to raise with your clinician.
I worked nights for years and have stopped. Does risk persist?
There is no reliable answer, because the human studies are not consistent enough to describe what happens after exposure ends. Ask your clinician which screenings are due for you.
Questions to ask your doctor
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Written by: Cancer ExplainedSources last checked: 2026-10-02 what this meansLast updated: 2026-08-10Next planned review: 2027-07-30
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How this page was created
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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