Night shift workers ‘at greater risk of breast cancer’

Research finds risk is strongest among 'morning' people

Women who work night shifts more than twice a week are at a greater risk of developing breast cancer, Danish research has found.

A study published online in the journal Occupational and Environmental Medicine shows that overall, night shift work is associated with a 40% increased risk of breast cancer compared with those who work regular hours.
 
But women who worked night shifts at least three times a week, and for at least six years, were more than twice as likely to have the disease as those who had not.

And those working this shift pattern were even more likely to develop breast cancer if they described themselves as 'morning' people rather than 'evening' people.

'Morning' types who work nights are almost four times as likely to have the disease than those who work regular hours, which the authors believe could be because such people are more susceptible to body clock disruption.

The findings showed that working up to two night shifts a week had no impact on the risk of developing breast cancer, as this may not be long enough to disrupt the body clock, say the authors.

On the other hand, they argue that frequent night shifts for several years may disrupt circadian rhythms and normal sleep patterns, and curb production of the cancer-protecting hormone melatonin, and therefore these factors may be linked to the development or progression of breast cancer cells.

The study was based on detailed questionnaires from 141 women with breast cancer, and 551 women without the condition, all of whom had worked for the Danish Army.

In 2007 the International Agency for Research on Cancer concluded that shift work, which disrupts the body clock, was “probably carcinogenic,” and called for more research.
 
Previous studies on the topic have focused on nurses, who might be exposed to other cancer causing agents, and have excluded other potentially influential factors, such as exposure to sunlight, according to the authors of this latest piece of research.

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