It was once thought far-fetched to suggest that smoking contributed to lung cancer. Now, another idea initially lacking in evidence 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
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 Centre. In 1987, Stevens
published a paper suggesting a link between light at night and breast
Back then, he was trying to figure out why breast cancer incidence
suddenly shot up starting in the 1930s in industrialised societies,
where night-time work was considered a hallmark of progress.
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 tumours and die quicker.
Some research has also shown that men working at night may have a
higher rate of prostate cancer.
Bigger studies needed
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 scepticism by some, but some doubters may reconsider when IARC publishes the results of its analysis, the result of an
expert panel convened in October, in the December issue of The Lancet
Evidence still limited
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 labelled 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.
Disrupts circadian rhythm
Scientists suspect that shift work is dangerous because it disrupts
the circadian rhythm, the body's biological clock. The hormone
melatonin, which can suppress tumour development, is normally produced
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 Centre
at Rensselaer Polytechnic Institute in New York, who is not connected
to IARC or its expert panel.
Compromises immune system
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.
Changing 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 travellers 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.
Working in red light
Some companies are also experimenting with different types of light,
hoping to create one that doesn't affect melatonin production. So far,
the colour 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
"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. – (Sapa-AP)