Almost a third of drivers have admitted momentarily nodding off at the wheel, according to a survey.
A total of 7% have actually fallen asleep while driving, the poll by road safety charity Brake and insurance company Direct Line found.
Also, 49% of the 1,000 drivers surveyed said they had driven after less than five hours of sleep.
The poll revealed that 31% have "head nodded", also known as micro sleeping, while at the controls, with 45% of male drivers owning up to this.
While only 2% of women drivers admitted falling asleep at the wheel, as many as 14% of male motorists confessed to this.
Men (55%) were also more likely than women (45%) to drive after less than five hours sleep.
Brake pointed out that research has shown that tired driving kills at least 300 people on UK roads every year.
Brake deputy chief executive Julie Townsend said: "The fact that so many drivers - especially men - have head-nodded at the wheel is horrifying, even more so that many don't recognise this means they have fallen asleep briefly.
"This survey suggests this is down to many people failing to ensure they always get sufficient sleep before embarking on journeys. We need all drivers to wake up to the fact that 'head nodding' is falling asleep, and can easily lead to catastrophe, but it can, of course be prevented. "
Direct Line's motor director Rob Miles said: "Tiredness and driving are a deadly combination. Regular breaks are essential for staying alert and awake, as is getting plenty of sleep the night before."
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