The beneficial effect of moderate-intensity activity on sleep quality depends, however, on the amount of exercise and the time of day it is performed, according to principal investigator Anne McTiernan, M.D., Ph.D., a member of Fred Hutchinson's Public Health Sciences Division.
McTiernan and colleagues found that women who exercised at a moderate intensity for at least half an hour each morning, seven days per week, had less trouble falling asleep than those who exercised less. Conversely, women who performed evening exercises experienced little or no improvement in sleep onset or quality.
"Postmenopausal women commonly report sleep problems. Exercise may help to alleviate these problems, as long as it is performed early in the day," says McTiernan, also a research professor in the School of Medicine (Division of Geriatrics) and School of Public Health and Community Medicine (Department of Epidemiology) at the University of Washington.
One possible explanation, the researchers note, is that morning versus evening exercise may differentially modulate circadian rhythms that affect sleep quality. More research is needed to confirm this theory.
While the study reports that increased physical fitness is a strong indicator of improved sleep quality in postmenopausal women, who are at higher risk for lower sleep quality, more research needs to be conducted to build on these preliminary findings as well.
The complete study, entitled "Effects of a Yearlong Moderate-Intensity Exercise and a Stretching Intervention on Sleep Quality in Postmenopausal Women," can be viewed on the journal SLEEP Web site, www.journalsleep.org.