Published on: February 27, 2015
by Salynn Boyles for MedPage Today:
Getting too little sleep each night has been linked to an increased risk for stroke, but sleeping too much may also raise stroke risk, according to results from two studies.
Long sleepers who averaged more than 8 hours of sleep each night were 46% more likely to have strokes than those who consistently slept 6 to 8 hours, researchers reported online in Neurology.
And those who transitioned from averaging less than 6 hours of nightly shut-eye to more than 8 hours had the highest risk, with close to a fourfold increase in stroke risk compared with people who consistently averaged 6 to 8 hours of sleep each night.
“We don’t know yet whether long sleep is a cause, consequence, or early marker of ill health. More research is needed to understand the relationship between long sleep and stroke,” said PhD candidate Yue Leng, of the University of Cambridge in England, in a written statement.
The study adds to the growing body of evidence on the ties between sleep and stroke risk.
A widely reported 2012 study found that less than 6 hours a night of sleep was associated with a more than fourfold increase in stroke risk. An earlier meta-analysis of sleep and stroke studies suggested a U-shaped relationship, with both short and long sleep associated with an increased risk for stroke. A more recent analysis of data from a large health study in China came to the same conclusion.
“Understanding this relationship is potentially important for the early detection of stroke, especially in older populations,” Leng and colleagues wrote.
Leng’s group conducted their own study of sleep duration and stroke risk in a British population, and they also updated the 2009 meta-analysis.
Their study included 9,692 mostly older adults (average age 61.6 at study entry) enrolled in a European cancer study who had never had a stroke and who were followed for an average of 9.5 years. Participants were asked about their sleep habits at baseline and then again 4 years later.
At baseline, 69% of participants reported sleeping 6 to 8 hours per night, while 10% reported sleeping for more than 8 hours and 21% reported sleeping less than 6 hours.
Those reporting less than 6 hours and more than 8 hours of sleep were more likely to be older and female, to be less active, and to have major depression. They also were more likely to take antihypertensive drugs.
Stroke risk among consistently long sleepers was double that of people who reported getting 6 to 8 hours of sleep, but a significant increase in risk associated with long sleep was only seen in participants who were age 63 or older (hazard ratio 1.50, 95% CI 1.09-2.05).
The risk was greatest among people who transitioned from less than 6 to more than 8 hours of sleep each night during the 4 years between sleep assessments (HR 3.75, 95% CI 1.17-12.05).
The study had some limitations, notably that the stroke-free participants were “younger and had higher social class and educational level compared to the baseline population,” the authors explained. Also, sleep duration was reported with a single question and that may reflect perception of sleep rather than biological sleep.
The meta-analysis included 11 studies in addition to the researchers’ own study, involving close to 560,000 participants from seven countries. Over a follow-up of 7.5 to 35 years, close to 11,700 strokes were reported.
Among the major findings, short sleepers averaging less than 6 hours of sleep a night had a pooled relative increased risk of 1.15 (95% CI 1.07-1.24, P=0.0002), with no evidence of heterogeneity.
Long sleep of more than 8 hours a night was associated with a pooled relative risk of 1.45 (95% CI 1.30-1.62), with significant between study heterogeneity (I2=54, P=0.003). When the analysis was repeated excluding a single study, this heterogeneity disappeared (RR 1.53, 95% CI 1.42-1.65; P<0.00001, I2=0%).
“This prospective study and meta-analysis suggested a significant increase in stroke risk among long sleepers and a modest increase among short sleepers,” the researchers wrote. “Prolonged sleep might be a useful marker of increased stroke risk in older people, and should be tested further for its utility in clinical practice.”
In an accompanying editorial, Alberto Ramos, MD, of the University of Miami, and James Gangwisch, PhD, of Columbia University in New York City, wrote that the observed association between long sleep duration and stroke, “poses a conundrum given the lack of evidence that sleeping for longer than 8 hours has adverse health effects.”
“Long sleep duration could be an epiphenomenon of stroke as opposed to a cause,” they wrote, adding that long sleep duration has been linked in cross-sectional studies with stroke risk factors such as inflammation, carotid artery atherosclerosis, and left ventricular mass.
“It is plausible that the associations between long sleep duration and the incidence of stroke in epidemiologic studies have been confounded by risk factors (i.e., sleep apnea, inflammatory markers) that were not measured and controlled for in multivariable analyses,” they wrote. “Future studies would ideally control for these factors, and determine if these explain or mediate the association between long sleep duration and stroke.”
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