But sleeping less than six hours a night was also linked to a modest increase in stroke risk.
The study compared sleeping patterns and stroke risk among 93,676 postmenopausal women aged 50-79 years. Over the seven-year study, 1,166 cases of stroke were recorded.
The lowest risk for stroke was seen in women who slept seven hours a night. Compared with this group, women whose average night's sleep lasted nine hours or more had a 70 per cent higher risk of stroke.
And women who slept six hours or less a night had a 14 per cent greater stroke risk.
But the researchers warned the findings only apply to postmenopausal women who may be more susceptible to the effects of sleep.
The apparent increase in stroke risk among those who slept for nine or more hours may be due to some unmeasured factors like undiagnosed sleep disorders, but further studies are needed, said lead researcher Dr Jiu-Chiuan Chen, from the University of North Carolina.
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