Driving for more than three hours without a break at night is the equivalent of being drunk behind the wheel, claims a new study.
Researchers found driver tiredness after a few hours has the same effect as being over the drink-driving limit.
Even two hours of motorway driving in the dark can affect performance so severely it is the same as having a couple drinks.
Now experts who carried out the research want governments to impose a maximum two-hour limit on continuous night-time driving in a bid to curb accidents and death rates.
It is estimated that one-fifth of all traffic accidents are due to sleepiness behind the wheel.
One in three motorists admits to nodding off while driving at night.
A study last year by experts at Cardiff University called for newly qualified drivers to be banned from the roads at night, a move that could save 200 lives a year in the UK and result in 1,700 fewer serious injuries.
Similar schemes already exist in New Zealand, Australia and the US.
Unlike alcohol, police have no way of measuring whether a driver is affected by fatigue.
To assess the extent to which tiredness hinders driving performance, researchers at Utrecht University in the Netherlands recruited 14 healthy young men aged 21 to 25.
Under supervision, each one drove for two, four and eight hours at a time through the night.
They had to maintain a constant 80mph on the motorway and remain in the centre of their traffic lane.
Researchers used video recordings to monitor the safety of their driving.
After the experiment, they compared motorists’ performances with what’s already known about the effect alcohol can have on driver safety.
The results, published in the Journal of Sleep Research, showed that after just two hours behind the wheel, the drivers were already making the same mistakes they would if they had 0.05pc blood alcohol content – more than half the UK legal drink drive limit of 80 mg of alcohol per 100 millilitres of blood.
At three hours, their performance corresponded to 0.08pc blood alcohol content – the national limit and by four-and-a-half hours it was equivalent to 0.10pc.
In a report on their findings, the researchers said: "Our data show that drivers should take sleepiness behind the wheel seriously. It is one of the primary causes of accidents on motorways.
"Yet drivers themselves are sometimes unaware of sleepiness, reduced alertness and corresponding impaired driving.
"Measures such as playing loud music or opening the window are of limited use. A maximum continuous night-time driving duration of two hours should be recommended."
Last year, a survey found that nearly three quarters of motorists (74pc) admitted driving while tired in the previous 12 months, with nearly one in 10 (nine percent) admitting to doing so once a week.
Experts estimate that one in five fatal crashes on trunk roads are caused by tired drivers, although it is believed that the figure could be higher, as it is difficult to establish whether a crash was caused by a driver falling asleep.
The survey found that despite official advice that motorists should take a break every two hours on long journeys, almost three quarters of those polled fail to do this, frequently driving for three hours or more at a time.
If motorists feel tired when driving they should pull over somewhere safe as soon as possible, drink caffeine and take a short nap.
Alternatively they should find somewhere to say overnight.