The study found that people who slept more than eight hours a night, called long sleepers, were 46 percent more likely to have a stroke than people who slept six to eight hours a night, which was considered an average amount of sleep. People who shifted over time from sleeping less than six hours a night to sleeping more than eight hours a night were nearly four times as likely to have a stroke as people who consistently slept an average amount.

The study involved 9,692 people with an average age of 62 who had never had a stroke. They were asked about their sleeping habits once and then again about four years later. The participants were followed for an average of 9.5 years. During that time, 346 people had a stroke.

Of the 986 people who slept more than eight hours a night, 52 had a stroke, compared to 211 of the 6,684 people who slept an average amount. The relationship between long sleep and stroke stayed the same after researchers accounted for factors such as high cholesterol, high blood pressure, body mass index and physical activity.

“We don’t know yet whether long sleep is a cause, consequence or early marker of ill health,” said study author Yue Leng of the University of Cambridge in the United Kingdom. “More research is needed to understand the relationship between long sleep and stroke.”

Alberto Ramos of the University of Miami Miller School of Medicine and a member of the American Academy of Neurology, who wrote a corresponding editorial, said, “Since people whose sleep patterns changed from short to long were nearly four times as likely to have a stroke, it’s possible that this could serve as an early warning sign, suggesting the need for additional tests or for people to take steps known to reduce stroke risk, such as lowering blood pressure and cholesterol.”

Leng and her colleagues also conducted a meta-analysis, which is a review of previous studies on sleep duration and stroke. Those results also found an association between long sleep and stroke.