But if sleeping too much is bad then so is sleeping too little, with getting less than 6 hours of sleep modestly increasing risk of stroke in postmenopausal women.
The study, led by Jiu-Chiuan Chen, M.D., Sc.D., assistant professor of epidemiology at the University of North Carolina's School of Public Health in Chapel Hill, showed that the risk of ischemic stroke was 60-70 percent higher for those sleeping nine hours or more than those sleeping for seven hours.
"After accounting for all common clinical conditions predictive of stroke, we found this increase was statistically significant: sleeping nine hours or more is strongly associated with increased risk of ischemic stroke," he said.
Researchers also found that women who slept six hours or less were at 14 percent greater stroke risk than those who slept seven hours a night.
Nearly twice as many women reported sleeping less than six hours (8.3 percent) than those who reported sleeping nine hours or more (4.6 percent).
"The prevalence in women of having long sleep duration is much lower than having sleep duration less than six hours. So the overall public health impact of short sleep is probably larger than long sleep," Chen said.
"This study provides additional evidence that habitual sleep patterns in postmenopausal women could be important for determining the risk of ischemic stroke," he added
The findings apply only to postmenopausal women and cannot be applied to other groups, including men and younger women. Postmenopausal women 50- to 79-years-old may be more susceptible to the detrimental effects of sleep deprivation than others.
"Our study population is a very specific group, and the duration of sleep needed for optimal health probably differs for various population groups," he said.
"People who feel that their sleep problems are a burden to their daytime activities are encouraged to discuss their concerns with physicians who can better interpret their significance and offer advice in the context of their overall health," he added.
Chen and his colleagues included known risk factors that might confound any apparent association of sleep duration and ischemic stroke in this analysis of data from the multi-ethnic Women's Health Initiative Observational Study. The study's 93,676 women were 50 to 79 years old at the time of their enrollment at 40 U.S. clinical centers.
Researchers conducted the study from 1994 to 2005.
Researchers asked the women how long they typically slept each night. The results were: five hours or less, 8.3 percent; six hours, 26.9 percent; seven hours, 37.5 percent; eight hours, 22.7 percent; nine hours, 4.0 percent and 10 hours or more, 0.6 percent.
After accounting for the common risk factors for ischemic stroke, the increased relative stroke risks compared to the seven-hour-sleep group were: 14 percent for six hours or less sleep; 24 percent for eight hours of sleep; and 70 percent for nine or more hours of sleep.