Women who work regularly at night face an increased risk of developing breast cancer, according to a new analysis published today.
And the problem seems to be linked to temperament, according to the study of women in the Danish military.
People who like to rise early in the morning seem to be at greatest risk from the disruption to their sleep patterns, facing a four times increased risk of developing disease, the researchers found.
Late risers fared better from working night shifts – with the risk of breast cancer just doubled.
The link is not new – but the researchers say their work reinforces the findings. Previous studies have involved nurses, who could also face increased risk because of chemicals they work with.
The study, reported in Occupational and Environmental Medicine, involved studying some 18,500 women studied over 35 years.
Researchers contacted some 200 women who developed breast cancer and another 900, free of the disease, to ask them about their work and shift patterns.
The research also showed that early-risers overall faced less risk of developing the disease than late-risers – provided they did not work regular shifts.
The researchers, led by Dr Johnni Hansen, of the Danish Cancer Society, say: "The findings indicated that working up to two night shifts a week had no impact on the risk of developing breast cancer, which may not be long enough to disrupt the body clock.
"On the other hand, frequent night shifts for several years may disrupt circadian rhythms and normal sleep patterns, and curb production of the cancer protecting hormone melatonin."
Occupational and Environmental Medicine May 29 2012; doi 10.1136/oemed-2011-100240
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