Regular physical activity can significantly lower a woman's risk of developing cancer, but skimping on sleep can eliminate those gains, a new study has found.

In a long-term study of nearly 6,000 US women, researchers found that those who exercised the most had a 25% lower chance of developing cancer than those who were the least active.

But among younger, physically active women, those who slept less than seven hours a night had a 47% higher risk of being diagnosed with cancer than those who regularly got a good night's rest.

"Greater participation in physical activity has consistently been associated with reduced risk of cancer incidence at several sites, including breast and colon cancers," said James McClain, a cancer prevention fellow at the National Cancer Institute and lead author of the study, released yesterday.

"Short duration sleep appears to have opposing effects of physical activity on several key hormonal and metabolic parameters, which is why we looked at how it affected the exercise/cancer risk relationship."

It is not yet known exactly why exercise reduces cancer risks but researchers believe it could be due to the lower body weight, improved immune function and hormone levels associated with regular physical activity.

Insufficient sleep has been linked to high risks of developing a number of conditions including heart disease, obesity and diabetes but, again, researchers have not determined exactly how sleep prevents disease.

The study was presented at a conference in Washington sponsored by the American Association for Cancer Research.