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Also, risk differences seen in 'morning' versus 'evening' people. By Steven Reinberg HealthDay Reporter MONDAY, May 28 (HealthDay News) -- Women who work the night shiftmore than twice a week might be increasing their risk for breastcancer, Danish researchers find. Moreover, the risk appears to be cumulative and highest among womenwho describe themselves as "morning" people rather than "evening"people, the researchers say. "About 10 to 20 percent of women in modern societies have nightshift work," said lead researcher Johnni Hansen. "It mighttherefore be one of the largest occupational problems related tocancer." Right now, the reasons for these findings are uncertain. "Night shift work involves exposure to light at night, whichdecreases the production of the night hormone melatonin that seemsto protect against certain cancers," said Hansen, of the Instituteof 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 fromlocal cellular clocks in different body organs, affecting thebreast, he said. "Repeated phase shifting may lead to defects in the regulation ofthe circadian cell cycle, thus favoring uncontrolled growth,"Hansen said. Also, sleep deprivation after night shift work leads to thesuppression of the immune system, which might increase the growthof cancer cells, he added. This is not the first time this association has been recognized. In2007 the International Agency for Research on Cancer, part of theWorld Health Organization, said that working the night shift is"probably carcinogenic to humans," according to backgroundinformation 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 breastcancer, Hansen's team collected data on more than 18,500 women whoworked for the Danish Army between 1964 and 1999. The researchers identified 210 women who had breast cancer andcompared them to almost 900 similar women who did not have breastcancer. All of the women were asked about their working patterns,lifestyles and other factors such as their use of contraceptivesand hormone replacement therapy, and their sunbathing habits. In addition, women were asked to classify themselves as "morning"or "evening" people. In all, 141 women with breast cancer responded to the studyquestionnaires. In addition, 551 women who did not have breastcancer responded. Among these women, the risk for breast cancer was increased 40percent if they worked at night, the researchers found. But for women who worked nights at least three times a week, andfor at least six years, the risk was doubled, the findings showed. Women who worked the night shift but who described themselves asmorning people were at even higher risk of breast cancer. They werealmost four times more likely to develop breast cancer as those whodidn't work nights, according to the researchers. In comparison, women who considered themselves evening people weretwice as likely to develop breast cancer, they added. Morning-preferring women who did not work at night had a loweroverall risk of breast cancer than evening types, Hansen's teamfound. "Since night shift work is unavoidable in modern societies, thistype of work should be limited in duration and limited to less thanthree night shifts per week," Hansen said. "In particular, morningtypes should limit their night work," he added. While the study found an association between night shift work andbreast cancer, it did not prove a cause-and-effect relationship. Men who work at night may also be at risk for prostate cancer,Hansen noted. This evidence comes from three small studies, hesaid. Dr. Stephanie Bernik, chief of surgical oncology at Lenox HillHospital in New York City, said that "it's very hard to single outthe causes of breast cancer and whether women on the night shifthave a higher risk." But there appears to be some truth to this, she added. "Stress increases the risk of breast cancer and affects the body asa whole, and working at night can throw off your circadian rhythmand cause stress," Bernik said. "This is another finding that breast cancer is caused by amultitude of different environmental and genetic influences, sothis is probably a piece of the puzzle," she added. An expert on environmental factors that affect cancer, RichardStevens, from the department of community medicine at the UConnHealth Center in Farmington, Conn., said that "the evidence isgrowing rapidly about light at night and, specifically, shift workand breast cancer." "But, this is the first study about the morning/eveningpreference," he noted. "If it's true that light at night increases the risk of disease,then there are very practical implications," Stevens said. If lighting is really an issue in night work, it is known whichwavelengths suppress melatonin the most, and lighting could beadjusted to eliminate those wavelengths, he explained. There are other things people can do to avoid the effects of lightat night on health, he added. "For example, for health in general, if you wake up during thenight, stay in the dark; don't turn on the light. If you turn onthe lights it will start suppressing melatonin immediately,"Stevens said. "There is a lot more involved than melatonin, but it's a goodmarker if your circadian rhythm is being changed," he said. Studies are ongoing on the risk of night work and prostate cancer,Stevens noted. More information For more about breast cancer, visit the American Cancer Society. SOURCES:Johnni Hansen, Ph.D., Institute of Cancer Epidemiology,Danish Cancer Society, Copenhagen; Stephanie Bernik, M.D., chief,surgical oncology, Lenox Hill Hospital, New York City;Richard G.Stevens, Ph.D., department of community medicine, UConn HealthCenter, Farmington, Conn.; May 28, 2012, Occupational and Environmental Medicine , online Copyright © 2012 HealthDay . All rights reserved. We are high quality suppliers, our products such as China Magnetic Floating Display , China Floating Bottle Display for oversee buyer. To know more, please visits Magnetic Floating Globe.
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