Last Updated: 2008-11-18 13:34:19 -0400 (Reuters Health)

Regular exercise can lower a woman’s overall risk of cancer, but only if accompanied by sufficient nightly sleep. A lack of sleep can undermine the cancer preventive benefits of exercise, new research shows.

Dr. James McClain, cancer prevention fellow at the National Cancer Institute in Bethesda, Maryland, reported his team’s findings Monday during the American Association for Cancer Research’s Seventh Annual International Conference on Frontiers in Cancer Prevention Research in Washington, D.C.

"Greater participation in physical activity has consistently been associated with reduced risk of cancer incidence at several sites, including breast and colon cancers," Dr. McClain explained in a written statement from the meeting. "Short duration of 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."

Their study involved 5,968 adult women with no previous cancer diagnosis. During 9.6 years of follow-up, 604 first-incident cancer cases were diagnosed, including 186 breast cancer cases.

"Consistent with previous studies," Dr. McClain noted, overall cancer risk and breast cancer risk were significantly reduced among women in the upper 50%, compared to the lower 50%, of physical activity energy expenditure. The hazard ratios were 0.80 and 0.75, respectively.

"Increased physical activity energy expenditure was protective for cancer irrespective of age," Dr. McClain and colleagues note in a meeting abstract.

However, the study also revealed a 1.5-fold increase in overall cancer risk (hazard ratio, 1.47) among women aged 65 or younger in the upper 50% of physical activity energy expenditure who did not have sufficient sleep (less than 7 hours per day).

These results suggest that "sleep duration modifies the relationship between physical activity and all-site cancer risk among young and middle-aged women," Dr. McClain told the conference. This "interesting finding" requires further study, he concluded.