Drivers who are just one or two hours shy of that recommendation nearly double their risk for a crash the next day, according to a new study conducted by the AAA Foundation for Traffic Safety (PDF) and released Tuesday.

"While we do not think anybody would be surprised to learn that driving while extremely sleep-deprived increases a driver`s risk of being involved in a crash -- this admittedly is rather intuitive -- we were surprised to find a detectable increase in crash risk when a driver had slept even just one hour less than the seven hours recommended by sleep experts," said Brian Tefft, senior research association for the foundation, who led the new study.

A report published by the Centers for Disease Control and Prevention in February found that more than a third of adults in the United States reported getting less than seven hours of sleep daily.

The CDC even has called insufficient sleep a "public health problem."

The new AAA study featured data on 7,234 drivers who were involved in 4,571 vehicle crashes, from 6 a.m. to midnight, between 2005 and 2007.

The data were from the National Highway Traffic Safety Administration`s National Motor Vehicle Crash Causation Survey, which included how much sleep drivers reported having in the 24 hours preceding a crash.

After analyzing the data, the researchers found that drivers who had slept for less than four hours had 11½ times the crash risk rate of drivers who had slept seven hours or more; drivers who had four to five hours of sleep had 4.3 times the risk; those who had five to six hours had 1.9 times the risk; and those who had six to seven hours had 1.3 times the risk.

In other words, "the crash risk of a driver who has slept for only four to five of the past 24 hours is approximately quadruple the risk of a driver who has slept for the expert-recommended minimum of seven hours, similar to the crash risk of a driver who is legally intoxicated relative to a sober driver," Tefft said.

A 2012 study published in the journal JAMA Internal Medicine found that sleepiness carried almost as much risk as alcohol ingestion while driving.

Another AAA study from 2010 found that as many as two out of five drivers have fallen asleep at the wheel at some point in their lives, he added.

"I have multiple friends and acquaintances who have fallen asleep at the wheel, including two who were involved in crashes as a result," Tefft said.

Tefft noted that the new study had some limitations, such as not including data on vehicle crashes between midnight and 6 a.m. and analyzing how only a lack of sleep in the past 24 hours was associated with crash risk, rather than quality of sleep.

"The study was designed specifically to investigate the relationship between acute sleep deprivation and crash risk," he said.

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