Driving drunk may be one of the worst hazards on the road, but driving drowsy is just as bad, according to a new study.
The research on shift workers says they're much more likely to fall asleep at the wheel.
New research from Boston's Brigham and Women's Hospital shows danger for those who drive after working the night shift. Nearly half the night-shift workers tested nearly crashed.
Dr Charles Czeisler worked on the study.
"I was shocked that nearly 40 percent of the drives had to have emergency braking manoeuvres in order to keep the car from going off the road," says Dr Czeisler.
"They were crossing over the middle line. They were having trouble. More than 40 percent of the drives had to be terminated because it was judged to be unsafe."
More than half the workers tested said they nodded off behind the wheel at least once a week on their way home, and they did no better in the tests.
"They knew they were being watched and yet even in that context, they couldn't stay awake while they were driving in a simulation of their commute home from work," says Dr Czeisler.
Dallas-area dairy manager Shariq Khan starts work at 11pm and works through the night.
"Sometimes when I get off from job, when I drive back home, I feel like I'm falling asleep," says Mr Khan.
He believes the lack of sleep led to him having a couple of minor accidents.
The researchers suggest night-shift workers like Mr Khan skip driving and find another way home, but his area lacks public transport.
"There's no alternative," he says. "Either I have to take a taxi or go by bus or train, which I can't; there's no bus or train service up there and I can't afford a taxi every day."
Researchers say the very nature of night-shift work fights the body's natural sleep rhythms. The average night-shift worker loses about two hours of sleep per night. After a few days, they have lost the equivalent of a full night's sleep.
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