"David," a Stanford professor, was biking in a race that lasted for several days. He got little sleep during the race, but after the race was over, he slept about nine hours for the next two nights.

Waking up on Sunday morning and feeling well rested, he started to drive home. But coming down the mountain road, he began to yawn and feel sleepy. He was surprised since he thought he was well rested.

Seeing a sign for a restaurant a few miles ahead, he figured he'd stop and get a copy of coffee.

But right after that he fell asleep, a microsleep lasting just a second or two. ( A microsleep is an unplanned sleep that can last just a second or two and you may not even know you fell asleep). He awoke to find he had drifted into the oncoming lane. He turned fast, but his maneuver didn't work.

He found himself upside down, suspended by his seat belt. He had dropped over a 30-foot ledge and was lucky to have his car impaled on a jagged rock.

David ended up with a completely paralyzed right arm, but at least he was alive.

Researcher William Dement, who invented the field of sleep medicine, tells this story in The Promise of Sleep, to illustrate the dangers of a big "sleep debt," the amount of sleep you have lost over several weeks and need to make up.

David had not repaid his sleep debt from the bicycle race and it caught up with him even though he felt fine at first.

Driving while sleeping can be just as dangerous as driving while drunk.

What should David have done when his eyelids got heavy? He should have pulled over and taken a nap.

A 30-minute nap would have made him smarter, faster, and far safer.

"Napping is by far the most important and effective tool for coping with sleep crises," Dement writes. He is the father of the field of sleep research and has studied the effects of select, strategic napping on performance and alertness.

A 45 minute nap, he has found, improved alertness for a full six hours after the nap.

A study of flight crew who took planned naps in the cockpit dramatically demonstrated the importance of naps, even for airplane pilots.

A NASA research team gave flight crews a planned 40-minute nap during each flight. ( A co-pilot was not asleep.) On other flights, the crew got no naps.

On flights where they did not nap, the crew went into a total of 120 dangerous microsleeps during the last 90 minutes of flight (descent and landing) and went into 22 microsleeps during the last 30 minutes of the flight.

In contrast, crews that napped took only 34 microsleeps during the last 90 minutes, and took zero microsleeps during the last 30 minutes.

You would be far safer with a flight crew that took planned naps.

Among the great daytime nappers were Albert Einstein, WInston Churchill, and presidents John Kennedy, Lyndon Johnson, and Bill Clinton, writes Jane Brody in a New York Times article.

Lyndon Johnson even put on pajamas for his nap.

But most Americans do not nap, especially women. On a typical day, about a third of adults (34%) take a nap, according to a nationally representative study conducted by the Pew Research Center in 2009.

In the United States, napping is stigmatized: "He got caught napping."

College-educated people, in demanding jobs, especially do not nap, found the Pew study.

But these college-educated people may be "outsmarting themselves," say the Pew Center researchers. They would be far more alert and productive if they did nap.

Plan for a daily nap, Dement urges. The best time is early afternoon, eight hours after you wake up. That's the time the body goes into a natural energy lull.

Even if you feel a little groggy after your nap, don't worry about it. Dement recommends a cup of coffee after your nap.

The grogginess will only last about fifteen minutes, and you'll be far brighter, safer, and more productive the rest of the day.