If you work the night shift more than twice a week, your risk for developing breast cancer just doubled.

That's the word from Danish researchers, who have determined that the risk is also cumulative and highest among night-shift workers who describe themselves as "larks" rather than "night owls."

Between 10 percent and 20 percent of women work the night shift. "It might therefore be one of the largest occupational problems related to cancer," said lead researcher Johnni Hansen of the Institute of Cancer Epidemiology at the Danish Cancer Society in Copenhagen.

The study: Information was collected on more than 18,500 women who worked for the Danish Army between 1964 and 1999. Of this group, Hansen's team conducted further interviews with 141 women who had breast cancer and compared them to 551 similar women who did not have breast cancer. The team asked them about their working patterns, lifestyle, sunbathing habits and use of contraceptives and hormone replacement therapy. Each woman was asked to classify herself as a "morning" person or an "evening" person.

The results: The risk of breast cancer doubled for those women who worked the night shift at least three times a week for at least six years. And among those night shift workers, those who described themselves as larks, had an even higher risk of developing breast cancer--specifically, four times the risk of those who didn't work at night. Women who considered themselves night owls were twice as likely to develop breast cancer.

Why? It's not exactly known, but Hansen suspects three variables are at play:

• Exposure to light at night decreases the production of the night hormone melatonin that seems to protect against certain cancers.

• Exposure to light at night can disrupt the body's circadian rhythm, a kind of master clock in the brain. Working different shifts--some at night and some during the day, back and forth--may lead to defects in the regulation of the circadian cell cycle, which can then favor uncontrolled growth of cells.

• Sleep deprivation after night shift work leads to the suppression of the immune system, which might increase the growth of cancer cells.

In 2007, the International Agency for Research on Cancer, part of the World Health Organization, found that working the night shift is "probably carcinogenic to humans."

Here's an interesting tidbit: Women who are larks--that is, those who prefer the morning hours--and did not work at night had a lower overall risk of breast cancer than did the night owls who worked days.

Important note: Although this research found an association between night shift work and breast cancer, it did not prove a cause-and-effect relationship.

The study findings were published in the journal Occupational and Environmental Medicine.

--From the Editors at Netscape