Stretching and moderate exercise can help overweight, postmenopausal women fall asleep more easily; but only if performed in the morning hours, according to a study published in the journal Sleep.
"Postmenopausal women commonly report sleep problems," says Anne McTiernan of Fred Hutchinson Cancer Research Center in Seattle. "Exercise may help to alleviate these problems, as long as it is performed early in the day."
The researchers found that women who exercised at a moderate rate for at least a half an hour each morning, seven days per week, had less trouble falling asleep than those who exercised less. Evening exercisers did not experience the same positive effects on sleep quality, though.
Lead author Shelley Tworoger, of Harvard University Medical School, says a possible explanation of the finding is that morning exercise could help set circadian rhythms, or the bodies' internal clock, which in turn regulates sleep-inducing hormones such as melatonin. "Or it could be that exercising at night increases body temperature, whereas body temperature must go down to induce sleep," she says. "But neither of these hypotheses have been proven."