Women who work night shifts are at greater risk of breast cancer especially if they consider themselves a 'morning person', research has found.
Women who work at least three night shifts a week for around six years or more are twice as likely to develop breast cancer, Danish research has found.
The effect was even greater in 'larks' who described themselves as morning people as they were four times more likely to develop breast cancer than women who did not work nights.
Women who said they were 'owl's and prefered to stay up late at night were less affected by night shifts but were still twice as likely to have breast cancer than those not working nights, it was found.
It is thought that disruption to the body clock and the resulting changes in levels of the 'darkness' hormone melatonin might be responsible along with sleep deprivation.
The study involved examining the medical records of 18,500 women who worked for the Danish Army between 1964 and 1999.