People who took a regular midday nap lasting more than 90 minutes were 25 percent more likely to later have a stroke than people who took a regular nap lasting from one to 30 minutes. People who took no naps or took naps lasting from 31 minutes to one hour were no more likely to have a stroke than people who took naps lasting from one to 30 minutes.
"More research is needed to understand how taking long naps and sleeping longer hours at night may be tied to an increased risk of stroke, but previous studies have shown that long nappers and sleepers have unfavorable changes in their cholesterol levels and increased waist circumferences, both of which are risk factors for stroke," said study author Xiaomin Zhang, MD, PhD, of Huazhong University of Science and Technology in Wuhan, China.
"In addition, long napping and sleeping may suggest an overall inactive lifestyle, which is also related to increased risk of stroke."
The study involved 31,750 people in China with an average age of 62. The people did not have any history of stroke or other major health problems at the start of the study. They were followed for an average of six years. During that time, there were 1,557 stroke cases.
The people were asked questions about their sleep and napping habits. Midday napping is common in China, Zhang said. Eight percent of the people took naps lasting more than 90 minutes. And 24 percent said they slept nine or more hours per night.
The study found that people who sleep nine or more hours per night are 23 percent more likely to later have a stroke than people who sleep seven to less than eight hours per night. People who sleep less than seven hours per night or between eight and less than nine hours per night were no more likely to have a stroke than those who slept from seven to less than eight hours per night.
The results were all adjusted for other factors that could affect the risk of stroke. These include high blood pressure, diabetes and smoking.
People who were both long nappers and long sleepers were 85 percent more likely to later have a stroke than people who were moderate sleepers and nappers.
The researchers also asked people about how well they slept. People who said their sleep quality was poor were 29 percent more likely to later have a stroke than people who said their sleep quality was good.
Of the long nappers, 1 percent of cases per person-years later had a stroke, compared to 0.7 percent of cases per person-years of the moderate nappers. The numbers were the same for the long and moderate sleepers, with 1 percent of cases per person-years compared to 0.7 percent of cases per person-years having a stroke.
"These results highlight the importance of moderate napping and sleeping duration and maintaining good sleep quality, especially in middle-age and older adults," Zhang said.
Zhang noted that the study does not prove cause and effect between long napping and sleeping and stroke. It only shows an association.
Limitations of the study include that information on sleep and napping was taken from questionnaires, not from recording people's actual sleep and information was not collected on sleep disorders such as snoring and sleep apnea. Also, the study involved older, healthy Chinese adults, so the results may not apply to other groups.