NEW YORK (Reuters Health) - Near-miss accidents should serve as a wake-up call to drowsy drivers that they run the risk of having an actual car crash, according to researchers.

In an Internet survey of more than 35,000 Americans, they found that the more "sleepy near-misses" respondents said they'd had, the greater the risk that they'd been in an actual crash.

Overall, the study found, 18 percent of respondents reported at least one drowsiness-related near-miss -- such as nodding off at a traffic light or drifting off onto the "rumble strips" on the shoulder of a highway.

Incidents like these are common, and many people can probably remember being in a similar situation, said lead study author Dr. Nelson B. Powell of Stanford University School of Medicine in California. However, if it has happened several times, he told Reuters Health, drivers should give it serious attention.

"The more near-misses you have, the higher the risk you'll have an accident," Powell said.

Drivers who've had several near-misses shouldn't be getting behind the wheel when they're sleepy, according to Powell. Moreover, they probably also need to re-evaluate their lifestyle, he said -- particularly whether they're getting enough hours of sleep every night.

The findings, published in the medical journal Sleep, are based on responses from 35,217 U.S. adults who responded to an Internet survey on driving habits and symptoms of daytime sleepiness.

Just over 1 percent said they'd had a sleepiness-related traffic accident in the last three years, but many more -- 18 percent -- had had a near-miss. Those who'd had four or more near-misses were 87 percent more likely to have had a real car wreck than respondents with no sleep-related near-misses.

"This is a precursor to an accident," Powell said.

Besides making lifestyle changes, he noted, drivers who find themselves chronically drowsy during the day should talk to their doctor about whether they might have a sleep disorder, such as sleep apnea.

Sleep apnea is a breathing disorder in which a person's breathing repeatedly stops and starts during the night, leading to poor sleep and daytime drowsiness.

In the current survey, people who said they had sleep apnea, insomnia or narcolepsy were all more likely to report sleep-related near-misses and actual accidents.