The short answer
Shift work that disrupts the body clock is classified as a probable carcinogen, with the most studied link to breast cancer. The evidence is still developing. Good sleep habits and lighting adjustments may help shift workers.
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.
It is most strongly linked to a possible link to breast cancer.
A carcinogen classification describes hazard — whether something can cause cancer — not your personal risk at a given exposure.
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The full explanation.
The simple version
Working nights or rotating shifts disrupts the body's internal clock, which some studies link to a modestly higher cancer risk, especially breast cancer. The evidence is not settled, and many things affect a shift worker's health. Small steps to protect sleep may help.
What night shift work is
"Shift work involving circadian disruption" means work schedules — especially night shifts — that regularly go against the body's natural day-night rhythm. IARC classifies it as probably carcinogenic (Group 2A), based mainly on studies of long-term night workers and supporting biology around melatonin and the body clock.
How people are exposed
Common ways people come into contact with it:
- Regular night-shift or rotating-shift work over many years
- Jobs in healthcare, transport, manufacturing, and emergency services
- Long-term disruption of normal sleep-wake cycles
The cancer connection
The most studied link is with breast cancer in long-term night-shift workers. Research into other cancers is ongoing, and results are mixed.
The International Agency for Research on Cancer (IARC), the cancer arm of the World Health Organization, places night shift work in Group 2A, probably carcinogenic to humans — meaning the evidence in people is limited but there is strong support from animal or mechanistic studies (evaluated in 2019).
Hazard is not the same as risk
It helps to separate two ideas that are easy to mix up: hazard and risk. When an agency lists night shift work as a carcinogen, it is making a statement about hazard — whether the substance is capable of causing cancer under some conditions. It is not, by itself, a statement about your personal risk, which depends on how much you are exposed to, for how long, and other factors. Two substances in the same group can carry very different real-world risks. The label answers "can it cause cancer?" — not "how likely is it to cause cancer for me?"
How to lower your exposure
- Protect sleep with a dark, quiet room and consistent routines
- Manage light exposure to support the body clock
- Keep up with recommended cancer screenings
- Support workplace scheduling that limits long-term night rotations
If you are looking at your overall cancer risk, small, steady steps add up. See our overview of cancer prevention and what raises cancer risk to put any single exposure in context.
The bottom line
Night shift work is a probable human carcinogen (IARC Group 2A). The most important thing you can do is understand where exposure comes from and take reasonable steps to reduce it, without losing sleep over a single label. Focus your energy on the biggest, most controllable risks in your own life.
Words to know
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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?
It is most strongly linked to a possible link to breast cancer. The evidence is still developing, and the possible effect appears modest compared with major risk factors.
▸How can I reduce my exposure to night shift work?
The main steps are protecting sleep and keeping up with screening.
▸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. Your actual risk depends on the amount and length of exposure and other factors.
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