Does working at night cause breast cancer?

'Graveyard shift' may actually be true

Warning: Working the night shift may cause cancer.

The International Agency for Research on Cancer (IARC), a World Health Organization agency based in Lyon, France, has designated working the night shift as a "probable" carcinogen. The finding is based on human and animal studies.

About 15 studies show a link between breast cancer and night shift work, indicating that women working the night shift may have a 30% to 80% increase in relative risk. That makes night shift work a potentially greater risk of cancer than second-hand smoke, which increases lung cancer risk by 25%, says Kurt Straif, MD, MPH, PhD, scientist, cancer epidemiologist for the monograph series.

The IARC has brought together working groups of scientific experts to evaluate evidence on the carcinogenicity of 900 agents. About 400 have been designated as carcinogenic or potentially carcinogenic.

Yet scientists want to see more research before classifying night shift work as a definite carcinogen. "The evidence in experimental animals was sufficient and in humans it was limited. There's some clear signal that there's an increased risk of breast cancer, but we could not rule out other explanations," says Straif.

A prospective study that followed 115,000 women for 12 years found an elevated breast cancer risk among women who worked more than 20 years of rotating night shifts.1 Another prospective study of women in the Nurses' Health Study found that working a night shift for at least three nights per month for 15 or more years was linked to an increased risk of colorectal cancer.2

Not every night shift worker seems to be at risk. "It seems to apply perhaps only to people who work long durations of shift work for extended periods of their lives," says lead author Eva S. Schernhammer, MD, DrPH, assistant professor of medicine at Brigham and Women's Hospital and Harvard Medical School in Boston.

Why would night work cause cancer? That's not entirely clear, but Schernhammer and her colleagues believe that it may have something to do with melatonin production, light exposure, and circadian rhythm.

"Many hormones are secreted throughout the day, but melatonin is secreted almost exclusively when it's dark. That's usually when we sleep," says Schernhammer. Light exposure at night suppressed the production of melatonin, she says.

Animal studies found that constant light or disruptions in circadian rhythm or the circadian period gene led to tumor formation.3

Role of melatonin explored

It's not clear whether melatonin itself is has a cancer-protective effect or whether it is a marker for the circadian rhythm, which may be an important factor, she says. Because that relationship isn't well-understood, Schernhammer and her colleagues do not recommend taking melatonin supplements.

Schernhammer noted that cancer isn't the only health risk associated with night shifts. "Night work has also been linked to a higher cardiovascular risk, higher rates of obesity, and other health outcomes that are not directly in the cancer pathway. It's fair to assume that this type of circadian disruption may actually cause a number of different [problems]," she says.

Health care workers and employers should be aware that night shift work may have health effects, says Straif. "Clearly, it sends a very strong signal that we should be concerned and as a precautionary principle we should not have unnecessary night shift work. We need more research to understand it," he says.

But Schernhammer puts it into perspective as just one more environmental exposure that can influence cancer risk. "There are many ways you can try to reduce your breast cancer risk," she says. "People can take other measures like losing weight or exercising more."

Meanwhile, researchers hope to gather more information about the cancer link. "My sense is that in five years from now, if we reconvene (as an IARC working group), we will have a much firmer answer to whether this moves up to being a definite carcinogen, or moves down to a potential carcinogen," she says.


1. Schernhammer ES, Kroenke CH, Laden F, et al. Night work and risk of breast cancer. Epidemilogy 2006; 17:108-111.

2. Schernhammer ES, Laden F, Speizer FE, et al. Night-shift work and risk of colorectal cancer in the nurses' health study. J Natl Cancer Inst 2003; 95:825-828.

3. Straif K, Baan R, Grosse Y, et al. Carcinogenicity of shift-work, painting and fire-fighting. Lancet Onco 2007; 8:1,065-1,066. Available at