"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 Centers for Disease Control and Prevention.
In the CDC study released Thursday, about 4 percent 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 government estimates that about 3 percent of fatal traffic crashes involve drowsy drivers, but other estimates have put that number as high as 33 percent.
Warning signs of drowsy driving: Feeling very tired, not remembering the last mile 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, a single second translates to speeding along for 88 feet — 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.