Work pattern affects night shift–breast cancer link

By Liam Davenport

The risk for breast cancer increases significantly with both the number of years of night shift work and the cumulative number of shifts, Danish researchers have discovered.

The effect was particularly pronounced in women who reported a preference for mornings over evenings and had a cumulative night shift work level over the median. These women were 3.9 times more likely to develop breast cancer than others

Night work, which is practiced by 10-20% of the workforce in Europe and the USA, disrupts circadian rhythms, suppresses melatonin production, and leads to sleep deprivation, all of which are linked to hundreds of metabolic and physiologic processes. Alteration of these processes may increase the initiation, progression, and growth of tumors.

To investigate further, Johnni Hansen and Christina Lassen (Danish Cancer Society, Copenhagen) identified over 18,000 female military employees born between 1929 and 1968.

There were 329 cases of primary breast cancer documented among these women between 1990 and 2003. Of these women, 218 were alive at the time of interview (March 2005 to December 2006) and 132 completed a 28-page lifestyle questionnaire. The questionnaire was also completed by 505 of the 899 age-matched healthy women who were used as controls.

The team reports in Occupational and Environmental Medicine that 43 women with breast cancer and 144 controls had a history of night shift work.

The overall adjusted odds ratio for breast cancer among participants who had ever done night shift work compared with those who had never done such work was 1.4.

The odds ratio increased to 1.7 among women who did night shift work for 6.0-14.9 years and to 2.1 in those who did it for 15 years or longer. A similar pattern was seen for cumulative night shift work exposure, with an odds ratio for breast cancer of 1.4 for the middle versus lowest tertile of cumulative exposure and 2.3 for the highest versus lowest tertile.

Of note, the researchers found that the increase in breast cancer risk was restricted to women who undertook night shift work at least 3 nights a week for at least 6 years.

The team writes: "This study gives further support to the hypothesis that night shift work increases the risk for breast cancer.

"The observation that women with night work and morning preference tend to have a higher risk for breast cancer than similar women with evening preference warrants further exploration."

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