WASHINGTON -- Working more than one job, getting less than six hours of sleep or driving in the wee hours increase the chance of nodding off behind the wheel, says the first study of what causes sleep-related crashes, which some experts believe account for 15 percent of all wrecks.
The study released Tuesday looked at 1,403 North Carolina drivers and found that among those who got into accidents last year, half had slept less than six hours the night before the crash. On average, they got 35 to 40 minutes less sleep per night than those who didn't get into accidents.
Drowsy drivers were almost twice as likely to be working more than one job and four to five times more likely to be working the night shift.
As is the case for many long-distance truckers, those in accidents were more likely to be driving alone and many had been awake for 20 or more hours when they crashed.
The National Highway Traffic Safety Administration estimates that 100,000 crashes -- 3 percent to 4 percent of accidents -- occur each year as a result of drivers falling asleep, causing 76,000 injuries and roughly 1,500 deaths.
But David K. Willis, president of the AAA Foundation for Traffic Safety, which sponsored the study, said most experts believe sleep deprivation may be to blame for up to 15 percent of crashes, including about 6,000 deaths.
"We hope the study that we are releasing today will be a wake-up call to all those folks who have not been getting enough sleep," he said.
Researcher Jane Stutts and colleagues at the University of North Carolina Highway Safety Research Center interviewed 467 drivers who were sleep-deprived or fatigued and involved in accidents, 529 drivers who crashed for other reasons, and 407 drivers who had not been in an accident in the past three years.
The researchers conducted telephone interviews shortly after the accidents and questioned the drivers about their work and sleep schedules, the quality of their sleep, any medications they might have taken that could cause drowsiness, and other factors. Although some drivers suffered from sleep disorders or were taking sleep-inducing medications, they were the minority, Stutts said.
"Almost half of the drivers we talked with said they felt only slightly or not at all drowsy before they crashed," she said. "I think the message here is that we can't always have a lot of warning before we fall asleep at the wheel."
Stutts said many of the drivers questioned took measures in an effort to keep themselves alert, like opening a window, turning up the air conditioning or playing music.
"This does not work to keep you awake," she warned. "The only truly effective action ... is to stop driving immediately, to pull over and get some rest, or let someone else take over the driving for you."
The only short-term remedy the researchers suggested is to stop driving, drink two cups of coffee and take a 15- to 20-minute nap before taking to the road again. Otherwise, the only true preventative measure is to get a full night's rest.
Jamie Summerlin, 29, of Rockwell, N.C., said she knew she was tired but thought she was fine to drive home after working a 12-hour shift at a plastics manufacturing company in May 1998.
"I hit a parked truck on the opposite side of the road and didn't even know it," she said. "You just like black out."
In October, six college students died in College Station, Texas, when a pickup truck driver fell asleep and struck a group heading to a fraternity party after a football game. Last month, six people were killed and four were injured in Mercury, Nev., when a minivan slammed into an oncoming car. Authorities believe the driver of the minivan may have fallen asleep.