Pink balloons are displayed in front of an artificial waterfall during the "Pink Ribbon" breast cancer awareness campaign at Cheonggye Stream in central Seoul October 5, 2011. The annual campaign is held to promote awareness of breast cancer and to support the fight against it. REUTERS/Jo Yong-Hak (SOUTH KOREA - Tags: HEALTH SOCIETY)

London: Women who work night shifts are at higher risk of breast cancer, warn researchers.

Their findings suggest that 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” or “larks” have a stronger risk than those who say they are “night owls”.

There has been mounting evidence that night shifts might boost cancer risk because of the disruption to the body clock and hormone production.

The latest study, backed by the Danish Cancer Society, involved more than 18 500 women working for the Danish army between 1964 and 1999. Researchers were able to contact 210 women out of a total of 218 who had breast cancer between 1990 and 2003.

These women were matched with 899 women who had also worked for the Danish army, but had not developed breast cancer.

Researcher Johnni Hansen said the findings suggested that working up to two nights a week was not long enough to disrupt the body clock. But frequent night shifts for several years may disrupt circadian rhythms – the body clock – and sleep patterns.

Exposure to light at night inhibits production of melatonin, which is produced by the pineal gland in the brain between the hours of 9pm and 8am. Melatonin, a hormone which dictates the natural cycles that govern sleep patterns, helps suppress tumours.

Research suggests that unusually low levels of melatonin, which are seen in people exposed to light during the night, may promote tumour growth. – Daily Mail