A new study associates daytime naps with an increased risk of stroke in elderly people.

Scientists at the Columbia University led an experiment involving about 2,200 people from Manhattan with an average age of 73 years, whose health conditions were monitored for two years.

The participants were asked to report how often they dozed off during the day and took a nap and also what they were doing when they fell asleep.

Forty-four percent of the participants reported that they did not doze off at all, meaning they never just fell asleep. About 47 percent reported that they dozed off on occasion, while 9 percent stated that they dozed off quite often.

Researchers were surprised to find that the risk of having a stroke went up by two-to four-fold in those who often fell asleep inadvertently during the day.

“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. 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," said lead-author Bernadette Boden-Albala of Columbia University in New York, according to Reuters.

Previous studies have shown that people who suffer from sleep apnea (short periods when breathing stops during sleep) have an increased stroke risk. Dr. Boden-Albala’s study is one of the first to look at the relationship between unplanned napping and “vascular events” such as stroke.

Dr. Boden-Albala said it is no 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.

Until other studies on these issues are ready, physicians are advised to assess their patients for sleep problems.

About 780,000 U.S. adults suffer a stroke each year, and about 150,000 of those will die. Around 150,000 people in the UK have a stroke each year.

The findings of the research were presented at the American Stroke Association’s International Stroke Conference held Thursday in New Orleans.