James McClain, Ph.D., cancer prevention fellow at the National Cancer Institute and lead author of the study, believes that physical activity has an effect on body weight, hormones and immune function, all of which contribute to a person's individual cancer risk. In addition, evidence is rapidly accumulating on the detrimental effects of insufficient and poor quality sleep. Sleep is becoming recognised as more critical to overall health and well-being than previously recognised, so much research is underway investigating the relationship between sleep quality, duration, and the risk of various diseases and conditions.
McLain says "Greater participation in physical activity has consistently been associated with reduced risk of cancer incidence at several sites, including breast and colon cancers. 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."
The study was conducted by evaluating the association between exercise, sleep duration and the incidence of cancer in general, breast cancer, and colon cancer in nearly 6,000 women over 18 years of age. The women completed surveys in 1998 and were tracked for nearly 10 years through registries in the US.
The results indicated that the duration of night-time sleep affects the relationship between exercise and reducing cancer risk in women. Women who exercised the most (those in the upper 50% in terms of physical activity) had a significantly reduced risk of overall cancer and breast cancer. However, among women up to age 65 who exercised the most, sleeping less than 7 hours per day nearly eliminated the protective effect of exercise. The researchers would next like to explore the possible reasons for these results, in the hopes of better understanding the roles of sleep and exercise in cancer prevention.