WASHINGTON - Less than half of Americans say they get a good night's sleep every night. Combine excessive sleepiness with a vehicle and the risk for a fall-asleep crash increases significantly. 

In fact, 28 percent of American drivers have admitted to falling asleep at the wheel, according to a recent National Sleep Foundation poll, and more than half (54 percent) said they have driven while drowsy. 

"People think they can judge the precise time they are too tired and don't realize that drowsy driving is a serious danger," said David Cloud, CEO of the National Sleep Foundation. "They don't know that it's possible to fall into a three- to four-second microsleep without realizing it. Traveling at 65 mph, that's enough time to travel the length of a football field basically unconscious." 

Even if a driver manages to stay awake, sleepiness causes slower reaction times, vision impairment, lapses in judgment and delays in processing information, which are all critical elements for safe driving practices. "Getting enough sleep can literally save your life," Cloud said. 

Drivers can prevent a fall-asleep crash by getting enough sleep the night before and by knowing the warning signs of sleepiness and using appropriate countermeasures. 

According to the National Sleep Foundation, if a driver starts to do the following, it's time to get off the road and find a safe place to pull over. 

• Have problems focusing, blink frequently and/or have heavy eyelids

• Drift from your lane, swerve, tailgate and/or hit rumble strips

• Have trouble remembering the last few miles driven

• Miss exits or traffic signs

• Have trouble keeping your head up

• Yawn repeatedly

• Find yourself rolling down the windows or turning up the radio. 

The National Sleep Foundation offers these countermeasures to prevent fall-asleep crashes: 

• Get a good night's sleep before you hit the road. You'll want to be alert for the drive, so be sure to get adequate sleep (seven to nine hours) the night before you go.

• Don't be too rushed to arrive at your destination. It's better to allow the time to drive alert and arrive alive.

• Use the buddy system whenever feasible for long distances. A buddy who remains awake for the journey can take a turn behind the wheel and help identify the warning signs of fatigue.

• Take a break every 100 miles or two hours. Do something to refresh yourself like getting a snack, switching drivers, or going for a run.

• Take a nap -- find a safe place to take a 15- to 20-minute nap, if you think you might fall asleep. Be cautious about excessive drowsiness after waking up.

• Avoid alcohol and medications that cause drowsiness as a side-effect.

• Avoid driving at times when you would normally be asleep.

• Consume caffeine. The equivalent of two cups of coffee can increase alertness for several hours.

For more information about drowsy driving prevention, visit the National Sleep Foundation's Web site at drowsydriving.org.