Women working two or more night shifts a week are at a greater risk of developing cancer.
The risk appears to be strongest among females who describe themselves as “morning people” or “larks”, rather than “evening people” or “owls”.
A Danish study published in Occupational and Environmental Medicine analysed a pool of 18,500 women who worked for the Danish Army between 1964 and 1999.
Of the 218 women that contracted breast cancer, 141 filled in a questionnaire on their jobs and working patterns.
Their answers were compared to 551 of the 899 women of the same age who also worked for the Danish Army.
It was found that night shift work was associated with a 40% increased risk of breast cancer compared with no night shifts.
Furthermore, women who worked night shifts at least three times a week for at least six years, were more than twice as likely to have the disease as those who had not.
The risk of breast cancer quadrupled among those who describe themselves as “morning people”, doubling for females calling themselves “evening types”.
Yet “larks” not working night shifts tended to have a lower overall risk of breast cancer than “owls.”
“Lack of sunlight has been linked to the development of various cancers, including that of the breast, but women who worked night shifts tended to sunbathe more frequently than those who worked during the day,” said the researchers, led by Dr Johnni Hansen, Institute of Cancer Epidemiology at the Danish Cancer Society.
“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.
“Such factors may be linked to the development or progression of breast cancer cells.”