The amount of sleep postmenopausal women get — either too little or too much — could affect their risk of having a stroke, suggests new research.

The study, conducted by researchers at the University of North Carolina, found that sleeping longer — nine hours to be exact — increased the risk of stroke in post-menopausal women by 60 to 70 per cent over those who slept seven hours.

"After accounting for all common clinical conditions predictive of stroke, we found this increase was statistically significant: sleeping nine hours or more is strongly associated with increased risk of ischemic stroke," said lead author Jiu-Chiuan Chen, assistant professor of epidemiology at the University of North Carolina's School of Public Health in Chapel Hill, in a release.

Sleeping less than six hours meant the women were 14 per cent more at risk of stroke than those sleeping seven hours.

The 93,676 women in the study — who were between 50 and 79 years old and enrolled at 40 U.S. clinical centres — were asked how many hours they slept at night.

The researchers said because women usually get less, rather than more, sleep, the risk of stroke stemming from sleep deprivation is likely of more concern than the one caused by oversleeping.

They say they are unsure as to why the longer sleepers were more at risk of stroke.

"The observed increase in stroke risk in long sleepers may be due to some unmeasured factors, such as undiagnosed sleep disorders, although we did attempt to account for that in our analysis."

Chen called for further study into the mechanisms affecting stroke risk.

The study, published Friday in the Journal of the American Heart Association, was conducted between 1994 and 2005.