Drivers who miss two or three hours of sleep in a day face the same risk of an accident as those who are legally drunk, according to an AAA study on the sometimes deadly consequences of being drowsy behind the wheel. The auto and travel group said Tuesday that its study of 4,571 U.S. accidents showed that drivers who got less than the recommended seven hours of sleep a day were at a much higher risk of accidents. The study by the AAA Foundation for Traffic Safety was the first to quantify the relationship between measures of sleep and the risk of getting into a crash, AAA said. "You cannot miss sleep and still expect to be able to safely function behind the wheel," David Yang, executive director for the AAA Foundation for Traffic Safety, said in a statement. Drivers who go without two to three hours of sleep in a 24-hour period more than quadrupled their risk of a crash, compared with those getting seven hours, the study said. That is the same crash risk that the U.S. National Highway Traffic Safety Administration has linked to driving with a blood alcohol level that's over the legal limit. Drivers who are short one to two hours of sleep nearly double their risk of having an accident, the AAA study showed. The Centers for Disease Control and Prevention has said that 35 percent of U.S. drivers sleep less than seven hours daily. A separate AAA study has found that about a fifth of fatal U.S. traffic accidents involve a sleep-deprived driver. Symptoms of drowsy driving can include having trouble keeping eyes open, drifting from lanes or not remembering the last few miles driven. But the AAA report said more than half of drivers involved in fatigue-related crashes experienced no symptoms. The AAA recommended driving while normally awake and taking a break every two hours or every 100 miles (160 kilometers). The report was based on figures from the National Highway Traffic Safety Administration covering accidents between July 2005 and December 2007.