A ten-year study of almost 6,000 women found that significantly fewer cancers occurred in those who were physically active. But sleeping less than seven hours a night wiped out the benefits of exercise and increased the risk of cancer.
"Current findings suggest that sleep duration modifies the relationship between physical activity and all-site cancer risk among young and middle-aged women," said research leader Dr James McClain, of the US National Cancer Institute.
Dr McClain's team assessed the effect of physical activity energy expenditure (PAEE) on the overall incidence of cancer, as well as their likelihood of developing breast or colon cancers.
All the women were aged over 18 and had no previous history of cancer. A total of 604 experienced a first incidence of cancer during the study period.
Women judged to be in the top 50 per cent PAEE bracket showed a significantly reduced overall risk of cancer. But for women aged under 65, getting less than seven hours sleep cancelled out much of the protective effect of physical activity.
The findings were presented yesterday at an American Association for Cancer Research international conference in Washington DC.
Dr McClain said the next step would be to investigate the link underlying the anti-cancer interaction between sleep and exercise.
Experts are not sure how exercise reduces cancer risk but believe hormone levels, immune function and body weight may all be involved.