Graveyard shift linked to cancer risk
Scientists suspect flipping body's light-dark cycle leaves workers vulnerable
LONDON - It was once scientific heresy to suggest that smoking contributed to lung
cancer. Now, another idea initially dismissed as nutty is gaining acceptance: the
graveyard shift might increase your cancer risk. Next month, the International Agency
for Research on Cancer, the cancer arm of the World Health Organization, will classify
shift work as a "probable" carcinogen.
That will put shift work in the same category as cancer-causing agents like anabolic
steroids, ultraviolet radiation, and diesel engine exhaust.
If the shift work theory proves correct, millions of people worldwide could be affected.
Experts estimate that nearly 20 percent of the working population in developed
countries work night shifts.
It is a surprising twist for an idea that scientists first described as "wacky," said Richard
Stevens, a cancer epidemiologist and professor at the University of Connecticut Health
Center. In 1987, Stevens published a paper suggesting a link between light at night and
breast cancer. Back then, he was trying to figure out why breast cancer incidence
suddenly shot up starting in the 1930s in industrialized societies, where nighttime work
was considered a hallmark of progress. Most scientists were bewildered by his
proposal.
But in recent years, several studies have found that women working at night for many
years are indeed more prone to breast cancer, and that animals who have their
light-dark schedules switched grow more cancerous tumors and die quicker. Some
research has also shown that men working at night may have a higher rate of prostate
cancer.
Because these studies have been done mainly in nurses and airline crews, bigger
studies in different populations are needed to confirm or disprove the findings.
The idea that shift work might increase your cancer risk is still viewed with skepticism
by some, but many doubters will likely be won over when IARC publishes the results of
its analysis, the result of an expert panel convened in October, in the December issue
of The Lancet Oncology.
The American Cancer Society said it would most likely add shift work to its list of
"known and probable carcinogens" when the IARC makes its reclassification. Up to
now, the society has labeled it an "uncertain, controversial or unproven effect."
Experts acknowledge the evidence is limited, but the "probable" tag means that a link
between shift work and cancer is plausible.
"The indications are positive," said Vincent Cogliano, director of the Monographs
program at IARC, which decides on carcinogen classifications. "There was enough of a
pattern in people who do shift work to recognize that there's an increase in cancer, but
we can't rule out the possibility of other factors."
The research suggests a correlation between people who work at night and increased
cancer rates. But the cause of the cancer might still be something else that people who
work at night do that is unaccounted for in the research.
Melatonin the prime suspect
Scientists suspect that shift work is dangerous because it disrupts the circadian
rhythm, the body's biological clock. The hormone melatonin, which can suppress tumor
development, is normally produced at night.
Light shuts down melatonin production, so people working in artificial light at night may
have lower melatonin levels, which scientists think can raise their chances of developing
cancer.
Sleep deprivation may also be a factor. People who work at night are not usually able to
completely reverse their day and night cycles. "Night shift people tend to be day shift
people who are trying to stay awake at night," said Mark Rea, director of the Light
Research Center at Rensselaer Polytechnic Institute in New York, who is not
connected to IARC or its expert panel.
Not getting enough sleep makes your immune system vulnerable to attack, and less
able to fight off potentially cancerous cells.
Confusing your body's natural rhythm can also lead to a breakdown of other essential
tasks. "Timing is very important," Rea said. Certain processes like cell division and
DNA repair happen at regular times. But if the body needs to do something at an
unusual time — like produce insulin in the middle of the night to help digest food — that
can set off a chain reaction of biological mistakes.
Flipping shifts even worse
Even worse than working the night shift would be to frequently flip between day and
night shifts.
"The problem is re-setting your body's clock," said Aaron Blair, of the United States'
National Cancer Institute, who chaired IARC's recent meeting on shift work. "If you
worked at night and stayed on it, that would be less disruptive than constantly changing
shifts."
Anyone whose light and dark schedule was frequently disrupted — including frequent
long-haul travelers or insomniacs — could theoretically face the same increased cancer
risks, Stephens said.
Scientists are now trying to figure out what might be possible to reduce shift workers'
risk of developing cancer. Melatonin can be taken as a supplement, but experts don't
recommend taking it long-term, since that could ruin the body's ability to produce it
naturally.
Some companies are also experimenting with different types of light, hoping to create
one that doesn't affect melatonin production. So far, the color that seems to have the
least impact on melatonin is one that few people would enjoy working under: red.
With no answers at the moment, experts say it's best to avoid shift work in the
long-term. But if that is impossible, there may be a simpler solution. "The balance
between light and dark is very important for your body," Stevens said, advising workers
to make sure they sleep in a darkened room when they get back from work.
"Just get a dark night's sleep," he said.
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Working at night linked to cancer risk?
