While the study found an association between night shift work and breast 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, he said.
Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City, said that "it's very hard to single out the causes of breast cancer and whether women on the night shift have 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 as a whole, and working at night can throw off your circadian rhythm and cause stress," Bernik said.
"This is another finding that breast cancer is caused by a multitude of different environmental and genetic influences, so this is probably a piece of the puzzle," she added.
An expert on environmental factors that affect cancer, Richard Stevens, from the department of community medicine at the UConn Health Center in Farmington, Conn., said that "the evidence is growing rapidly about light at night and, specifically, shift work and breast cancer."
"But, this is the first study about the morning/evening preference," 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 which wavelengths suppress melatonin the most, and lighting could be adjusted to eliminate those wavelengths, he explained.
There are other things people can do to avoid the effects of light at night on health, he added.
"For example, for health in general, if you wake up during the night, stay in the dark; don't turn on the light. If you turn on the lights it will start suppressing melatonin immediately," Stevens said.
"There is a lot more involved than melatonin, but it's a good marker if your circadian rhythm is being changed," he said.
Studies are ongoing on the risk of night work and prostate cancer, Stevens noted.
For more about breast cancer, visit the American Cancer Society.
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