Alcohol, smartphones and distractions often lead to car accidents, but a surprising number of crashes are related to drowsy drivers, according to a new study.

Of fatal collisions, 21 percent involve drivers who are too sleepy to focus on the road, the Washington Post reported via the AAA Foundation for Traffic Safety.

"This new research further confirms that drowsy driving is a serious traffic safety problem," Peter Kissinger, president and CEO of the foundation, said in a statement. "Unfortunately, drivers often underestimate this risk and overestimate their ability to combat drowsiness behind the wheel."

The study, which used a sample of 14,268 crashes that occurred in years 2009-'13, found that a third of accidents involving a drowsy driver result in injuries, while each year more than 6,000 crashes related to tired driving result in at least one death.

"Despite the fact that 95 percent of Americans deem it 'unacceptable' to drive when they are so tired that they have a hard time keeping their eyes open, more than 28 percent admit to doing so in the last month," Kissinger said. "Like other impairments, driving while drowsy is not without risk."

The AAA Foundation has found in earlier research that drivers ages 19-24 are the most likely to admit to tired driving, with 33 percent of young adult drivers reporting being tired behind the wheel in the last month.  On the other hand, drivers ages 75 and older were the least likely to report tired driving along with the youngest group of drivers, ages 16-18.

The nonprofit warns drivers to beware of signs of drowsy driving, which include disconnected thoughts, a heavy head, problems with focusing, repeated yawns and difficult staying in the correct lane. 

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