But don't worry about a nap or two. "This is significant dozing" -- nodding off frequently and unintentionally while watching TV or reading during the day, says researcher Bernadette Boden-Albala, PhD, of Columbia University in New York.
Previous research has shown that people who have sleep apnea, brief periods when breathing stops during sleep, are at increased risk of stroke.
But Boden-Albala says this is one of the first studies to look at the association between unplanned daytime dozing and stroke.
The researchers studied more than 2,100 adults with an average age of 73. They were asked how often they fell asleep during specific daytime situations, such as watching TV, sitting and talking to someone, or stopping in traffic.
Based on their responses, 9% were classified as being significant dozers, 47% as moderate dozers, and 44% as no-dozers.
Over the next two years, there were 127 vascular events such as strokes, heart attacks, or deaths from them.
Among the findings, presented at the American Stroke Association's (ASA) International Stroke Conference:
The risk of stroke was 2.6 times greater for moderate dozers than for people who didn't unintentionally fall asleep during the day.
Significant dozers had a 4.5-fold higher risk of stroke, compared with no-dozers.
The risk of a heart attack or dying of vascular disease was also higher -- 1.6% for moderate dozers and 2.6% for significant dozers.
So just what makes someone a significant dozer? According to Boden-Albala, "This might be someone who always falls asleep while watching TV, always falls asleep while sitting and reading, and sometimes falls asleep while sitting on the couch.
"But it's cumulative, so someone who falls asleep about 50% of the time during all three situations" would also be a significant dozer, she says.
If you have trouble staying awake throughout the day, talk to your doctor, Boden-Albala says.