Getting less than six hours of sleep a night on a regular basis increases the risk of stroke among normal-weight adults, according to new research.

Researchers at the University of Alabama at Birmingham studied 5,666 people who had no history of stroke, stroke symptoms or high risk for obstructive sleep apnea for up to three years.

They recorded the first stroke symptoms with demographic information, stroke risk factors, depression symptoms and various health behaviours.

Once the results were adjusted to account for body-mass index, the researchers found that there was a strong link between daily sleep amounts of less than six hours and a greater incidence of stroke symptoms for middle-aged to older adults.

There was no association between short sleep periods and stroke symptoms for overweight and obese participants in the study.

"In employed middle-aged to older adults, relatively free of major risk factors for stroke such as obesity and sleep-disordered breathing, short sleep duration may exact its own negative influence on stroke development," the study's lead author Megan Ruiter said in a release. "We speculate that short sleep duration is a precursor to other traditional stroke risk factors, and once these traditional stroke risk factors are present, then perhaps they become stronger risk factors than sleep duration alone."

The study is being presented Monday at SLEEP 2012, the annual meeting of the Associated Professional Sleep Societies.