CHICAGO (Reuters) - Older people who have significant trouble staying awake during the day have more than four times the normal risk of having a stroke, U.S. researchers said on Thursday.

They also found a higher risk of heart attacks and other cardiovascular problems in seniors who regularly nod off during the day without planning to.

"Even when we controlled for things like hypertension, diabetes, physical activity, obesity and socioeconomic status, we found that people who had significant daytime dozing were much more likely to end up with stroke," said Bernadette Boden-Albala of Columbia University in New York.

Her study of 2,153 adults with an average age of 73 found the risk of stroke was 2.6 times greater for those classified as doing "some dozing" during the day compared to those with "no dozing." Those in the "significant dozing" group had a 4.5 times greater risk.

The study, presented at an American Stroke Association conference in New Orleans, is one of the first to look at the relationship between unplanned napping and "vascular events" such as stroke.

Other studies have found that people with sleep apnea who briefly stop breathing throughout the night are at high risk of stroke. This can cause daytime sleepiness.

Boden-Albala's study involved mostly Hispanic men and women over 40 living in the same community in New York City. None had suffered a stroke.

The researchers asked people to rate how often they dozed in specific situations, such as watching TV, sitting quietly after a lunch without alcohol and stopping briefly in traffic while driving. Some 44 percent reported no dozing, 47 percent had some dozing and 9 percent reported significant dozing.

After 2 1/2 years, they checked to see how many had strokes or other vascular problems, such as a heart attack. They detected 40 strokes and 127 other vascular events.

UNCERTAIN CAUSE

They found those who had the most trouble staying awake had the highest stroke risk, but the biggest surprise was in the moderate dozing group.

"We found that group was also significantly associated with about a 2.5-fold increased risk of stroke and about a 60 percent increased risk of having any kind of vascular event," Boden-Albala said in a telephone interview.

She said it is not clear what is causing the daytime sleepiness or if there is a link with sleep apnea.

"Whether it is sleep deprivation or sleep apnea, physiological changes are occurring that may be related to this increased risk. I think we may need to investigate that further," she said.

A separate study at the conference found moderate aerobic fitness helped protect people from stroke, even if they had other risk factors such as heart disease or diabetes.

The study, which involved more than 60,000 people, is the first to single out the benefits of aerobic fitness on stroke prevention.

"We found that a low-to-moderate amount of aerobic fitness for men and women across the whole adult age spectrum would be enough to substantially reduce stroke risk," said Steven Hooker of the University of South Carolina.

About 780,000 U.S. adults suffer a stroke each year, and about 150,000 of those will die.