Women who routinely work graveyard shifts, such as nurses, have a
substantially higher risk of developing breast cancer, according to two new
studies.
U.S. researchers found that working, on average, one all-night shift a week
over a three-year period increased a woman's risk of developing cancer by 60
per cent compared to a woman who has never toiled nights.
The risk increased further with each additional hour per week of
graveyard-shift work, with women who worked three days a week seeing their
risk more than double, according to research published in today's edition of
the Journal of the U.S. National Cancer Institute.
Scott Davis of the Fred Hutchinson Cancer Research Center in Seattle said
that the culprit seems to be light, and its effect on the hormone melatonin.
Melatonin production peaks during sleep. Researchers theorize that exposure
to light at night interrupts the output of melatonin, which in turn
stimulates a woman's ovaries to produce extra estrogen. Excess production of
the female sex hormone is a risk for breast cancer.
A second study, also published today in the NCI Journal, independently found
similar results.
Researchers at Sloan-Kettering Cancer Center in New York studied the work
histories of more than 78,000 nurses over a 10-year period. They found that
those who worked nights three times a month for as little as one year were
about 8 per cent more likely to develop breast cancer, while those who
worked more occasional night shifts for their entire careers saw their risk
increase as much as 36 per cent.
The idea that too much exposure to light can raise a woman's cancer risk
derives from earlier research on blind women, who are half as likely to
develop breast cancer as sighted women. In blind women, melatonin levels do
not fluctuate and, as a result, their estrogen levels are more stable.
Dr. Davis said that while the evidence looks compelling, it would be
premature to make any specific recommendations about limiting graveyard
shifts.
The reason, he said, is that factors other than light may influence breast
cancer risk. For example, night shift workers tend to work in extremely
high-risk professions such as nursing, firefighting and law enforcement. It
is possible that stress boosts estrogen production independent of exposure
to light, Dr. Davis said.
The researchers plan to do follow-up work comparing the hormone levels of
day-shift and night-shift nurses.
There have been, over the years, a plethora of studies on the health impact
of working the graveyard shift. These workers have higher rates of
everything from heart disease to colds, and many of the problems are linked
to sleep deprivation.
Last year, an estimated 19,200 Canadian women were diagnosed with breast
cancer and 5,500 died.