Studies from 2001 to 2005 found that working late hours on a regular basis could increase the chance for women to develop breast and colon cancer and men to develop prostate cancer. But after 20 years of collecting data, Judith Schwartzbaum, an associate professor of epidemiology at Ohio State University and lead author of the study, concluded that such a link wasn't possible.
“Many of these studies included very specific worker populations,” she said. “For example, studies of female flight attendants have found an increased risk of breast cancer and also a higher-than-expected risk for developing malignant melanoma.
“But airline workers differ from other shift workers due to their increased exposure to cosmic and solar radiation,” Schwartzbaum added. “So it’s tough to tease out what exactly may contribute to their elevated risk of cancer.”
The researchers collected information from 1971 through 1989 of Swedish citizens who worked at least 20 hours a week in 1970 – a total of about 3.2 million people. However, only 4 percent of men and 0.4 percent of women had jobs that met the definition of shift work. For men, the main night-shift occupations included working in the paper manufacturing industry, working as furnace operators and working as firefighters, policemen or train operators. The occupations of women who worked at night primarily included crane or hoist operators, delivery agents in the paper and publishing industries or midwives.
In previous studies that had linked late-shift work with cancer, researchers said that the connection could have been due to a decrease in the production of the hormone melatonin, as some animal experiments suggest that the hormone may have anti-cancer properties.
"Our bodies produce their highest levels of melatonin at night, during sleep, but exposure to light at night suppresses melatonin production," Schwartzbaum said. But she emphasized that the effects of melatonin on cancer development in humans were “not well understood.”
She also stated that large-scale international studies were needed to help discover the relationship between shift work and the risk of developing cancer.