At least the risk for a stroke increases, according to a publication by Leng et al. in Neurology, 2015: Sleep duration and risk of fatal and nonfatal stroke: A prospective study and meta-analysis [Leng2015]. They studied the association between sleep duration and stroke incidence in a British population.
More than 9k study participants were asked about their habits of sleeping, afterwards the authors of the study recorded who of them had a stroke, for about 9 years.
This study showed, that participants who sleep more than 8 hours a day are 46% more likely to get a stroke than those who sleep 6-8 hours:
Those with short sleep had an 18% increased stroke risk (not statistically significant), while long sleep was associated with a 46% increase in stroke risk after adjustment for conventional cardiovascular disease (CVD) risk factors and comorbidities.
It’s not yet clear whether there is a causal connection or just a correlation, but I found that potentially interesting. Is a longer sleep already a sign for a bad health condition? Even worse, is sleep a reason for a stroke?
Also read: Is sleep duration a risk factor for stroke? [Ramos2015]