July 18, 2008 — (BRONX, NY ) — Postmenopausal women who regularly sleep more than nine hours a night may increase their risk of ischemic stroke, researchers reported in Stroke: Journal of the American Heart Association today. Ischemic strokes involve decreased blood flow to the brain.
The research was conducted as part of the Women's Health Initiative (WHI), the largest multi-site longitudinal study looking at health risks among postmenopausal women.
Compared to women sleeping seven hours, the risk of ischemic stroke was 60 to 70 percent higher for those sleeping nine hours or more, said lead author Dr. Jiu-Chiuan Chen, assistant professor of epidemiology at the University of North Carolina's School of Public Health in Chapel Hill.
"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 had a 14 percent greater risk of stroke compared with 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).
"What we don't know is whether the longer sleep time was the reason for the increased risk or whether there was some other factor that both led people to sleep more and was also a risk factor for stroke," said Dr. Sylvia Wassertheil-Smoller, principal investigator of the WHI site located at Albert Einstein College of Medicine of Yeshiva University, one of 40 sites nationwide and the only one in New York City. "In other words, this study does not mean that if you cut your hours of sleep you would lower your stroke risk."
The findings involve only postmenopausal women and may not apply to other groups, such as men and younger women. Postmenopausal women 50- to 79-years-old may be more susceptible to the detrimental effects of sleep deprivation than others.
Three large studies have previously addressed the role of sleep duration in coronary artery disease and/or stroke. They yielded mixed results and didn't account for the many factors that may increase the risk of cardiovascular disease and stroke, including age, race, socioeconomic factors such as family income and employment, smoking, depression, exercise, hormone therapy in postmenopausal women, high-blood pressure, high cholesterol, diabetes, and obstructive sleep apnea.
Drs. Chen, Wassertheil-Smoller and their 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 the 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 controlling 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. Among the team's other results:
Long sleep durations were associated with being retired or unemployed; smoking, being physically inactive or having cardiovascular disease, diabetes, hypertension, high cholesterol or depression.
Overweight women and minority women were more likely to sleep six hours or less. Women with current hormone therapy were less likely to sleep six hours or less. Income, physical activity, depressive symptoms and other cardiovascular risk factors correlated with sleep duration but didn't fully account for the link between stroke and sleep patterns.
"People who sleep excessively long hours habitually — or who sleep less than six hours habitually — should discuss this with their doctors and be sure to lower their other risk factors for stroke, especially high blood pressure," said Dr. Wassertheil-Smoller.
Medical, psychosocial and lifestyle variables included in their analysis could not completely explain the increased ischemic stroke risk associated with short and long sleep among postmenopausal women. But Dr. Chen said many studies have documented physiological consequences of sleep deprivation.
"Our data do not imply that if women with long sleep cut their sleep hours they would be at a lower risk," Chen emphasized. "At this point, we still cannot determine that long sleep causes ischemic stroke. The observed increase in stroke risk in long sleepers may be due to some unmeasured factors, such as undiagnosed sleep disorders, although we did attempt to account for that in our analysis."
Co-authors are Robert L. Brunner, Ph.D.; Hong Ren, M.S.; Joseph C. Larson, M.S.; Douglas W. Levine, Ph.D.; Matthew Allison, M.D.; Michelle J. Naughton, Ph.D. and Marcia L. Stefanick, Ph.D.