Like UV rays and diesel exhaust fumes, working the graveyard
shift will soon be listed as a "probable" cause of cancer.
It is a surprising step validating a concept once considered
wacky. And it is based on research that finds higher rates
of breast and prostate cancer among women and men whose
work day starts after dark.
Next month, the International Agency for Research on
Cancer, the cancer arm of the World
Health Organization, will add overnight shift work
as a probable carcinogen. The American
Cancer Society says it will likely follow. Up to
now, the U.S. organization has considered the work-cancer
link to be "uncertain, controversial or unproven."
The higher cancer rates don't prove working overnight
can cause cancer. There may be other factors common among
graveyard shift workers that raise their risk for cancer.
However, scientists suspect that overnight 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.
If the graveyard shift theory eventually 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.
Among the first to spot the night shift-cancer connection
was 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 over many years were indeed more
prone to breast cancer. Also, animals that have their
light-dark schedules switched develop more cancerous tumors
and die earlier.
Some research also suggests that men working at night
may have a higher rate of prostate cancer.
Because these studies mostly focused on nurses and airline
crews, bigger studies in different populations are needed
to confirm or disprove the findings.
There are still plenty of skeptics. And to put the risk
in perspective, the "probable carcinogen" tag means that
the link between overnight work and cancer is merely plausible.
Among the long list of agents that are listed as "known"
carcinogens are alcoholic beverages and birth control
pills. Such lists say nothing about exposure amount or
length of time or how likely they are to cause cancer.
Cancer Society Web site notes that carcinogens
do not cause cancer at all times.
Still, many doubters of the night shift link may be won
over by the IARC's analysis to be published in the December
issue of the journal Lancet Oncology.
"The indications are positive," said Vincent Cogliano,
who heads up the agency's carcinogen classifications unit.
"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."
Scientists believe having lower melatonin levels can
raise the risk of developing cancer. Light shuts down
melatonin production, so people working in artificial
light at night may have lower melatonin levels.
Melatonin can be taken as a supplement, but experts don't
recommend it long-term, since that could ruin the body's
ability to produce it naturally.
Sleep deprivation may be another factor in cancer risk.
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 with the IARC analysis.
Not getting enough sleep makes your immune system vulnerable
to attack, and less able to fight off potentially cancerous
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.
Even worse than working an overnight shift is flipping
between daytime and overnight work.
"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
Anyone whose light and dark schedule is often disrupted
including frequent long-haul travelers or insomniacs
could theoretically face the same increased cancer
risk, Stevens said.
He advises workers to sleep in a darkened room once
they get off work. "The balance between light and dark
is very important for your body. Just get a dark night's
Meanwhile, scientists are trying to come up with ways
to reduce night workers' cancer risk. And some companies
are experimenting with different lighting, seeking a type
that doesn't affect melatonin production.
So far, the color that seems to have the least effect
on melatonin is one that few people would enjoy working