Sleeping a lot may increase the risk for stroke, a new study has found.

Chinese researchers followed 31,750 men and women whose average age was 62 for an average of six years, using physical examinations and self-reported data on sleep. They found that compared with sleeping (or being in bed trying to sleep) seven to eight hours a night, sleeping nine or more hours increased the relative risk for stroke by 23 percent. Sleeping less than six hours a night had no effect on stroke incidence.

The study, in Neurology, also found that midday napping for more than 90 minutes a day was associated with a 25 percent increased risk for stroke compared with napping 30 minutes or less. And people who both slept more than nine hours and napped more than 90 minutes were 85 percent more likely to have a stroke.

The study controlled for smoking, drinking, exercise, family history of stroke, body mass index and other health and behavioral characteristics.

The reason for the association is unclear, but long sleep duration is associated with increased inflammation, unfavorable lipid profiles and increased waist circumference, factors known to increase cardiovascular risk.