A study released by the AAA Foundation for Traffic Safety and the University of North Carolina Highway Safety Research Center (HSRC) shows that drowsy driving crashes are strongly related to sleeping less than six hours per night.
Other factors associated with having a drowsy driving crash included being awake for 20 hours or longer, working more than one job, working night shifts, and frequent driving between midnight and 6 a.m.
The AAA Foundation and HSRC studied hundreds of sleep and fatigue-related car crashes to identify the driver behavior that caused them.
"Previous studies about drowsiness and driving were done in a laboratory," said David K. Willis, president of the AAA Foundation. "This is the first time anyone has looked at real-world crashes to see what happens."
The study used police crash reports and driver records to identify and interview 1,400 drivers. The sample included four groups: drivers who fell asleep, drivers who were fatigued, drivers who crashed for non-sleep reasons and a control group of drivers who had not had a crash in three years.
The researchers administered a detailed questionnaire about the drivers' work schedules, sleep habits, quality of sleep, amount of driving and the circumstances surrounding their crash.
Results showed that sleep and fatigue crash drivers had been awake longer and had slept less compared with nearly half of the control group.
Drivers in sleep and fatigue crashes were also more likely to try to deal with their drowsiness once they were on the road.
The study indicated that many drivers do not know how sleepy they are. Around half the drivers in sleep-related crashes said they did not feel even moderately drowsy before they crashed.
The full study may be found on the AAA Foundation's Web Site at www.aaafoundation.org