MONDAY, May 28 (HealthDay News) -- Women who work the night shift more than twice a week might be increasing their risk for breast cancer, Danish researchers find.
Moreover, the risk appears to be cumulative and highest among women who describe themselves as "morning" people rather than "evening" people, the researchers say.
"About 10 to 20 percent of women in modern societies have night shift work," said lead researcher Johnni Hansen. "It might therefore be one of the largest occupational problems related to cancer."
Right now, the reasons for these findings are uncertain.
"Night shift work involves exposure to light at night, which decreases the production of the night hormone melatonin that seems to protect against certain cancers," said Hansen, of the Institute of Cancer Epidemiology at the Danish Cancer Society, in Copenhagen.
In addition, light at night might introduce circadian disruption, where the master clock in the brain becomes desynchronized from local cellular clocks in different body organs, affecting the breast, he said.
"Repeated phase shifting may lead to defects in the regulation of the circadian cell cycle, thus favoring uncontrolled growth," Hansen said.
Also, sleep deprivation after night shift work leads to the suppression of the immune system, which might increase the growth of cancer cells, he added.
This is not the first time this association has been recognized. In 2007 the International Agency for Research on Cancer, part of the World Health Organization, said that working the night shift is "probably carcinogenic to humans," according to background information in the study.
The new study was published in the May 28 online edition of Occupational and Environmental Medicine.
To determine the effect of night shift work on the risk for breast cancer, Hansen's team collected data on more than 18,50
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