The study, reported at the American Stroke Association's International Stroke Conference 2008, showed that stroke risk was two- to four-fold greater in those with moderate dozing. Bernadette Boden-Albala, Ph.D., an assistant professor of neurology at Columbia University's College of Physicians and Surgeons in New York City, lead author of the study, said the findings suggest that daytime dozing 'may be an important and novel stroke risk factor.'
In this study, dozing means a person unintentionally falling asleep. The researchers found that among 2,153 participants in a prospective study with an average follow-up of 2.3 years, the risk of stroke was 2.6 times greater for those classified as doing 'some dozing' compared to those with 'no dozing.' Those in the 'significant dozing' group had a 4.5 times higher risk.
"Those are significant numbers. We were surprised that the impact was that high for such a short period of time," Boden-Albala said.
Previous research has shown that people who suffer from sleep apnoea - short periods when breathing stops during sleep - have an increased stroke risk.
It could be that daytime sleepiness is a sign of sleeping poorly at night because of sleep apnoea.
For the study, Boden-Albala and colleagues studied a community-based cohort as part of the long-term Northern Manhattan Study (NOMAS), which began in 1990 and included men and women ages 40 and older.
This was the first effort investigating stroke risk factors in whites, blacks and Hispanics living in the same community.
No study participants had suffered a stroke. At the beginning of the study, their average age was 73 years and 64 percent were women. The racial-ethnic mix was 60 percent Hispanic, 20 percent black and 18 percent white.
In 2004, the researchers began collecting daytime dozing data annually using the Epworth Sleepiness Scale, which asks people to rate their frequency of dozing off during specific situations, such as watching TV, sitting and talking to someone, sitting quietly after a lunch without alcohol and stopping briefly in traffic while driving.
Based on the scale findings, the researchers designated participants as 'no dozing' (44 percent), 'some dozing' (47 percent) and 'significant dozing' (9 percent).
In the two years of follow-up, Boden-Albala and colleagues sought to determine the number of strokes and vascular events, which they defined as a heart attack or stroke death caused by vascular problems, among the dozing study members. They detected 40 strokes and 127 vascular events.
After taking into account several stroke risk factors such as age, race-ethnicity, sex, education, blood pressure, diabetes, obesity and physical activity, the researchers found high stroke risks for the 'some dozing' and 'significant dozing' groups as compared to 'no dozing.'
They found that the risk of a heart attack or vascular death was higher, 1.6 percent for the moderate dozers and 2.6 percent for the significant dozers.
The results were similar for all ethnicities and both genders.
These findings, if confirmed by other studies, carry important public health implications as well.
"Studies demonstrate that we are not getting enough sleep, so we're tired. But the real question is, what are we doing to our bodies? Sleepiness obviously puts us at risk of stroke," Boden-Albala said.