One percent of U.S. drivers - 1.9 million people - have had accidents or near-misses over the past year because they were driving while sleepy, a new survey finds.

Also, more than half of drivers surveyed admitted that they''e driven over the last year while feeling drowsy, and 28 percent said they drive while sleepy at least once a month, according to the survey sponsored by the National Sleep Foundation.

"People underestimate how tired they are and think that they can stay awake by sheer force of will," Thomas Balkin, chairman of the National Sleep Foundation, said in a news release about the 2009 Sleep in America poll. "This is a risky misconception. Would there be 1.9 million fatigue-related crashes or near misses if people were good at assessing their own ability to drive when fatigued?" he asked.

"The problem is that although we are pretty good at recognizing when we feel sleepy, we do not recognize the process of actually falling asleep as it is happening," Balkin said. "The process robs us of both self-awareness and awareness of our environment. All it takes is a moment of reduced awareness to cause a crash."

Research has suggested that not sleeping for more than 20 hours can make people as impaired as if they were drunk. Sleepiness slows reaction times, makes people less aware and impairs their judgment, researchers say.

What do the experts suggest? The National Sleep Foundation recommends that you watch out for these warning signs:

1. Difficulty focusing, heavy eyelids and difficulty avoiding daydreams
2. Nodding, forgetting the last few miles you've driven, and repeated yawning
3. Restlessness and irritability