Adults sleeping for more than eight hours per night may face a significantly higher risk of stroke, according to a recent study. Sleeping less than six hours may also increase the risk of stroke, but not by as much.

A nine-and-a-half year University of Cambridge study of almost 10,000 adults aged 42 to 81 found that those who slept for more than eight hours per night were 46 percent more likely to suffer a stroke. Interestingly, those who slept for less than six hours showed an 18 percent greater likelihood of stroke.

Researchers warned, however, that they did not know whether sleeping longer was a cause, effect or early warning symptom of cardiovascular problems. However, lead author of the study, Yue Leng PhD candidate at the University of Cambridge, warns that individuals should not try to sleep less in order to reduce their risk of stroke, as a good night’s rest is critical to overall health.

“We need to understand the reasons behind the link between sleep and stroke risk. What is happening in the body that causes this link,” stated Professor Kay-Tee Khaw of the Cambridge Institute of Public Health, one of the study authors. “With further research, we may find that excessive sleep proves to be an early indicator of increased stroke risk, particularly among older people.”

The study followed 9,692 stroke-free adults that were part of the European Prospective Investigation into Cancer (EPIC)-Norfolk cohort study. When the study began, the average age of participants was 62. Seven in 10 participants reported sleeping between six and eight hours, while 10 percent said that they slept more than eight hours per night.

After almost 10 years, 346 participants suffered a stroke, with those who slept over eight hours at 46 percent greater risk. Researchers also found that participants whose sleep patterns changed from less than six to more than eight hours per night were at even higher risk of stroke.

While those who slept less than six hours also showed a greater risk of stroke, study authors said that the number of relevant cases was too small to draw a firm conclusion.

Researchers also noticed some interesting patterns. Study participants who slept less than six or more than eight hours per night were older, female, and not active. Additionally, the relationship between stroke risk and sleep duration was stronger for fatal strokes than non-fatal strokes.

Furthermore, those who slept less than six hours were at greater risk for ischemic stroke, while long sleepers were more at risk for hemorrhagic stroke. Finally, sleeping more than eight hours increased the risk of stroke for individuals over 63 years of age more than for younger people, while short sleep increased the risk of stroke for younger people.

As part of the study, researchers additionally combined data from 11 other studies dealing with the relationship between sleep and stroke. The Researchers’ final analysis included 560,000 participants and supported their findings from the EPIC-Norfolk cohort study.

While several studies have found a relationship between oversleeping and other major health problems like obesity and diabetes, this is the first study to analyze sleep duration and the risk of stroke.

Too little sleep has been associated with increased levels of cortisol (the “stress hormone”) and a disrupted metabolism, both of which have been linked to high blood pressure and higher stroke risk. However, even after adjusting for common stroke risk factors like high blood pressure, gender and age, researchers found an independent correlation between longer sleep and stroke risk.

“It’s apparent both from our own participants and the wealth of international data that there’s a link between sleeping longer than average and a greater risk of stroke,” said Leng in a press release. “What is far less clear, however, is the direction of this link, whether longer sleep is a symptom, an early marker or a cause of cardiovascular problems.”

In an editorial accompanying the study, Dr. Alberto Ramos, assistant professor of neurology at the University of Miami Miller School of Medicine, suspects that sleeping longer may be a warning signal of cardiovascular problems.

“Long sleep duration could therefore be a harbinger of stoke through its association with potent cardiovascular risk factors,” wrote Dr. Ramos. “Lengthening sleep duration could portend strokes and serve as an early warning sign, suggesting the need for further diagnostic testing or for taking precautionary measures.”