Women who work night shifts are at higher risk of breast cancer, according to new research.
In fact, it suggests working at night increases the chances of the disease by 40 percent.
Women working more than two night shifts a week have double the risk of those on day shifts, says a report from scientists, while night workers who also describe themselves as "morning people" have a stronger risk than those who say they are "night owls."
The latest study, backed by the Danish Cancer Society, involved more than 18,500 women working for the Danish army between 1964 and 1999. They based their finding on a detailed questionnaire they gave the women about their habits.
Overall, night shift work was linked with a 40 percent increased risk of breast cancer compared with no night shifts. Women who had 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.
There has been growing evidence that night shifts might boost cancer risk because of the disruption to the body clock and hormone production.
Experts believe a hormone in the body that potentially suppresses tumors may be disrupted by constant exposure to light during night-time hours.
Shift work can increase the likelihood of other lifestyle risk factors, such as lack of exercise.