LONDON - 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. The American 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 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.
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 shifts."
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 sleep."
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 under: red.
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. The American 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 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.
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 shifts."
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 sleep."
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 under: red.
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