Old people who often fall asleep inadvertently during the day could have more than four times the normal risk of having a stroke, U.S. researchers said Thursday.

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.

A team from New York's Columbia University monitored the health of 2,153 patients with an average age of 73 for more than two years.

Participants were asked to report how often they dozed off during specific situations such as watching TV, having a conversation and stopping briefly in traffic while driving.

Based on these responses, participants were categorized as "no dozing" (44 percent), "some dozing" (47 percent) and "significant dozing" (9 percent). Dozing was defined as unintentionally falling asleep.

In 2.3 years of follow-up, people classified with "some dozing" were 2.6 times more likely to have a stroke than "no dozers," while "significant dozers" were 4.5 times more likely to have a stroke.

The authors suspected that stroke and dozing off may be linked via a condition known as sleep apnea.

According to previous studies, daytime sleepiness is associated with various sleep disorders including sleep apnea.

But Professor Jim Horne, director of the Sleep Research Centre at Loughborough University, said daytime dozers should not necessarily worry.

"In a few cases sleepiness may be linked to a stroke," he said. "But the most obvious cause of daytime sleepiness is disrupted sleep at night, especially for elderly people."

(Agencies via Xinhua February 22, 2008)