This could give you nightmares: 1 in 24 U.S. adults say they recently fell asleep while driving.

And health officials behind the study think the number is probably higher. That's because some people don't realize it when they nod off for a second or two behind the wheel.

Drivers should get off the road if they feel very tired, don't remember the last kilometre or two, or drift onto rumble strips on the side of the road. (Reed Saxon/Associated Press)

"If I'm on the road, I'd be a little worried about the other drivers," said the study's lead author, Anne Wheaton of the U.S. Centers for Disease Control and Prevention.

In the CDC study released Thursday, about 4 per cent of U.S. adults said they nodded off or fell asleep at least once while driving in the previous month. Some earlier studies reached a similar conclusion, but the CDC telephone survey of 147,000 adults was far larger. It was conducted in 19 states and the District of Columbia in 2009 and 2010.

CDC researchers found drowsy driving was more common in men, people ages 25 to 34, those who averaged less than six hours of sleep each night, and — for some unexplained reason — Texans.

Wheaton said it's possible the Texas survey sample included larger numbers of sleep-deprived young adults or apnea-suffering overweight people.

Most of the CDC findings are not surprising to those who study this problem.

"A lot of people are getting insufficient sleep," said Dr. Gregory Belenky, director of Washington State University's Sleep and Performance Research Center in Spokane.

The U.S. government estimates that about 3 per cent of fatal traffic crashes involve drowsy drivers, but other estimates have put that number as high as 33 per cent.

Warning signs of drowsy driving: Feeling very tired, not remembering the last kilometre or two, or drifting onto rumble strips on the side of the road. That signals a driver should get off the road and rest, Wheaton said.

Even a brief moment nodding off can be extremely dangerous, she noted. At 97 km/h, a single second translates to speeding along for nearly 27 metres — the length of two school buses.

To prevent drowsy driving, health officials recommend getting 7 to 9 hours of sleep each night, treating any sleep disorders and not drinking alcohol before getting behind the wheel.

In a 2007 survey, about 60 per cent of Canadian drivers admitted they occasionally got behind the wheel while fatigued and 15 per cent admitted they'd fallen asleep while driving in the past year, according to Transport Canada's website.