Night Shift Work and Cancer

In June 2019, 27 independent experts from 16 countries met in Lyon, France. They worked 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 conclusion: night shift work is probably carcinogenic to humans, Group 2A.

That is not a change. An IARC Working Group reached the same classification in 2007. It was published as Volume 98 in 2010. So why meet again? Because a great many new studies appeared in the decade that followed. An IARC advisory group convened in 2014 gave shift work high priority for re-evaluation between 2015 and 2019. The full 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.

The Working Group reviewed four separate kinds of data. Where and how people are exposed to night shift work. Studies of cancer in exposed people. Studies of cancer in laboratory animals exposed to altered light–dark schedules. And studies of the mechanisms by which the exposure might act.

Four cancer sites had the most information behind them: breast, prostate, colon, and rectum. Positive links were seen at all four. Some studies covered specific worker groups, such as nurses, factory workers, and airplane cabin and cockpit crew. Others drew on the general population, covering anyone who had held a job with night work.

Then comes the sentence that many summaries skip. Some large, informative studies found no link between night shift work and breast cancer, or any other cancer type. Other large, informative studies did find one. The Working Group's explanation is about method. The new studies used many different ways to measure night shift exposure, some better than others. So the group concluded that a positive link exists overall for breast, prostate, colon, and rectal cancer. It also concluded that bias could not be ruled out with reasonable confidence.

That is an honest description of a real state of knowledge. It is neither reassurance nor alarm.

The evidence in laboratory animals is graded sufficient, the stronger term. What was tested there was not a shift roster. It was the underlying exposure: alteration in the light–dark schedule.

The mechanistic evidence was rated strong in experimental systems. IARC named the three effects it rested on. Immunosuppression, meaning a weakened immune response. Chronic inflammation. And increased cell proliferation, meaning more cell division. Those three show up across many carcinogens. Finding them here is what let a limited human signal combine with animal data into a 2A.

The most pronounced effect of night shift work is the disruption of circadian rhythms. Those are the daily cycles that govern normal body function. IARC is careful on the last step. It says plainly that it is unknown to what extent those disruptions lead to cancer.

About 20% of workers in North America, Europe, and elsewhere work outside the standard daytime shift. Some sectors run far higher. Manufacturing, medical work, retail, services including information technology, and transportation all do.

Some of that work happens in one place: a factory, a restaurant, a hospital ward. Some happens on an aircraft crossing time zones. Night shift work 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.

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.

Two agents in the same group can carry very different real-world risk. The type and extent of exposure differ. So does the size of the effect at a given exposure level. IARC makes this point bluntly in its own Q&A for this volume. The classification puts night shift work in the same category as working as a hairdresser or barber. And it says directly that exposures sharing a category are not therefore equally dangerous. Our page on hazard versus risk works through that distinction, and how the IARC monographs work covers how the categories are assigned.

IARC also declines two things people often expect from it. It does not make recommendations about what an individual should do after exposure to a possible carcinogen. Its Q&A suggests consulting a physician instead. It also does not recommend regulations or legislation. Those remain the responsibility of national governments, though health and regulatory agencies do use Monographs evaluations when deciding what to do.

For someone who has worked nights for years, the useful conclusions from Volume 124 are narrow but real.

The four cancer sites named are the ones worth raising with a clinician. Three of them have established screening pathways that apply to the general population anyway. Screening schedules for breast, colon, and rectal cancer do not change because of shift work. But a night roster is a common reason people fall behind on appointments, and that part is fixable. See breast cancer and colorectal cancer for what those pathways involve.

The exposure tested in animals was light at the wrong time, not tiredness. That is worth knowing. It points at lighting, sleep timing, and shift rotation patterns as the things under study, rather than at effort or fatigue.

And the honest headline is this. A decade of new research left the classification exactly where it was in 2007. More studies did not settle the question. They mapped it more carefully.

• IARC — IARC Monographs Volume 124: Night Shift Work
• IARC — Night Shift Work, Questions and Answers, PDF
• IARC — Monographs evaluation of the carcinogenicity of night shift work
• PubMed — Carcinogenicity of night shift work, Lancet Oncol 2019, PMID 31281097