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 U.S. 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.

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.