Regular exercise can reduce a woman's risk of cancer, but the benefits may slip away if she gets too little sleep, U.S. researchers said on Monday.

The study involving 5,968 women in Maryland confirmed previous findings that people who do regular physical activity are less likely to develop cancer.

But when the researchers looked at the women ages 18 to 65 who were in the upper half in terms of the amount of physical exercise they got per week, they found that sleep appeared to play an important role in cancer risk.

Those who slept less than seven hours nightly had a 47 percent higher risk of cancer than those who got more sleep among the physically active women, the researchers reported at a meeting of the American Association for Cancer Research.

"We think it's quite interesting and intriguing. It's kind of a first look into this. It isn't something that has been widely studied," James McClain of the National Cancer Institute, part of the U.S. government's National Institutes of Health, said in a telephone interview.

McClain, who led the study, said it is unclear exactly how getting too little sleep may make one more susceptible to cancer. "Getting adequate sleep has been long associated with health," McClain said.

The U.S. Centers for Disease Control and Prevention calls sleep loss an under-recognized public health problem, saying Americans are getting less and less slumber. The CDC said the percentage of adults reporting sleeping six hours or fewer a night increased from 1985 to 2006.

Sleep experts say chronic sleep loss is associated with obesity, diabetes, high blood pressure, stroke, cardiovascular disease, depression, cigarette smoking and excessive drinking.

In addition, research had shown that people who get regular exercise have a reduced risk of breast, colon and other types of cancer. Experts think the effects of exercise on the body's hormone levels, immune function and body weight may play an important role.