Stress is generally considered neither pleasant nor healthy, but researchers now say that women who suffer a lot of it on a daily basis have lower rates of breast cancer.

The finding results from an 18-year study in Copenhagen of more than 6,500 women, who were asked at the beginning what levels of stress they experienced on a routine basis. The symptoms that defined stress were tension, nervousness, impatience, anxiety or sleeplessness.

At the end of the 18 years, 251 women had developed breast cancer for the first time. Naja Rod Nielsen of the National Institute of Public Health in Copenhagen and colleagues write in the British Medical Journal that those who had reported high levels of stress were 40% less likely to have developed breast cancer than those who categorised their stress levels as low.

The more stress women reported, the less likely they were to get the disease: for every increased level of stress on a six-grade scale, women were 8% less likely to develop breast cancer.

The researchers have a tentative explanation for their findings: They say severe stress over a long period can affect oestrogen levels - and over time it is possible this could have an influence on the disease. High levels of oestrogen are associated with a greater breast cancer risk.

But they warn that this theory had not been tested, and the only relevant research has been done on animals.

The women's stress levels were not measured during the study, and the charity Breakthrough Breast Cancer sounded a cautionary note.

"It's always hard to measure the impact of stress on breast cancer risk as it is difficult to untangle from other factors in our lives, and everyone views their own stress levels differently," said Sarah Rawlings, the head of policy at the charity.

"This study doesn't help us to draw further conclusions. However, maintaining a healthy, balanced lifestyle is important - we know that high stress levels can lead to unhealthy behaviour, which may alter your risk of breast cancer and other diseases."