AACR: Exercise and Poor Sleep Combine as Odd Cancer Risk
WASHINGTON, Nov. 19 -- Although exercise has generally been found to prevent cancer, combining it with poor sleep may be a risk factor, a researcher said here.
Among 4,033 women younger than 65 who had at least 20 hours of physical activity per week, those who slept less than seven hours nightly had a 47% higher risk of developing cancer (95% CI 5% to 105%) within the next 10 years, reported James J. McClain, Ph.D., of the National Cancer Institute.
He emphasized that the increased cancer risk associated with poor sleep was seen only in women with relatively high levels of physical activity. Moreover, in the entire sample, frequent exercise was protective (HR 0.80, 95% CI 0.68 to 0.95).
"These findings highlight the importance of being physically active with respect to reducing cancer risk and suggests that sleep, specifically duration, is also important particularly in those women who are very physically active," Dr. McClain said at the American Association for Cancer Research's Frontiers in Cancer Prevention Research meeting.
"This interesting finding needs to be confirmed in subsequent studies," Dr. McClain added.
It emerged from the ongoing CLUE II cohort study of 5,968 women in Washington County, Md., who were free of cancer and completed a questionnaire on exercise frequency and sleep duration in 1998.
At that time, the median level of reported physical exercise in the cohort was 1,164 minutes per week for women younger than 65, and 772 minutes per week among the 1,935 participants 65 and older.
About two-thirds of women in both age groups reported getting an average of at least seven hours of sleep per night.
With 9.6 years of follow-up, 604 women in the cohort subsequently were diagnosed with cancer, including 331 of those under 65 and 273 of the older women.
Dr. McClain reported a significant (P=0.04) trend toward lower cancer risk with exercise among those younger than 65.
Women in the third-highest quartile of physical activity in 1998 had a nonsignificant hazard ratio of 0.92 (95% CI 0.68 to 1.24) for developing cancer, relative to women in the lowest quartile.
Those in the highest activity quartile had a hazard ratio of 0.72 (95% CI 0.52 to 1.00).
Dr. McClain said that when sleep duration was compared with cancer incidence in each age group, there was no significant relationship seen.
But among those with above-median exercise in 1998, those getting less than seven hours of sleep per night showed a significantly greater rate of cancer.
Statistically, the interaction between sleep, exercise, and cancer risk was significant (P=0.015), he said.
He cautioned that this was the first study to correlate sleep duration, exercise, and cancer risk simultaneously and hence was "very preliminary."
"Based on what we know about physical activity and short duration of sleep, mechanisms related to inflammation or DNA damage could be of interest," Dr. McClain said.
He said these would have to be explored in future research.
Lucile Adams-Campbell, Ph.D., a cancer epidemiologist at Georgetown University in Washington, noted that sleep disruption is common in women undergoing chemotherapy for cancer.
She suggested that the findings reported by Dr. McClain may support research into interventions to improve sleep in these patients.
Dr. McClain agreed, saying, "It would be particularly interesting to look at some of these questions [on sleep, exercise, and cancer risk] in populations of survivors."
