Copenhagen (dpa) - A new Danish study shows that women who work at night have a sharply increased chance of getting breast cancer, the Copenhagen newspaper Berlingske Tidende reported on Friday.

"Women who have had night work for more than six years, increase their risk of breast cancer by 70 per cent," Johnni Hansen, a researcher attached to the Danish cancer society Kraeftens Bekaempelse was quoted as saying.

Hansen presented during a conference on health and safety at the work place on Tuesday the findings of a nationwide survey of more than 7,000 women with breast cancer born in the 1935-59 period.

All the women were aged 30-54 when they got cancer, and their working conditions were compared with those of an equally large group of women who did not get breast cancer.

The study showed a higher risk of breast cancer for night workers regardless of the occupation, and the risk increased in proportion to how long the women had worked the night shift.

Even just six months' night work seems to raise the breast cancer risk, according to the study.

But researchers so far have no definite explanation for why female night workers, such as nurses, stewardesses, brewery and restaurant employees have an increased risk of breast cancer.

One theory is that night work especially disturbs sleep cycles and the hormonal balance in the body.

Another is that the accumulated stress reaction from shift work has a negative effect on workers' health.

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