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This could give you nightmares: One 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.

"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 four 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.

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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.

Although the study is based solely on U.S. data, a similar situation likely occurs in many other countries, including Canada.

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

The warning signs of drowsy driving are: 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 60 mph, drivers travel 27 metres – the length of two school buses – in a single second.

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

A major problem is that some sleep disorders – particularly sleep apnea – are not diagnosed and go untreated.

Sleep apnea occurs when the soft tissues in the throat collapse shut. Those with the condition must wake briefly many times a night to gulp for air. They will have no recollection of waking, but suffer the consequence of poor sleep – chronic fatigue. Sleep apnea is treated with a CPAP (continuous positive airway pressure) machine that blows air down the throat to keep the passageway open.

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