It's that dangerous moment that drivers fear most: nodding off at the wheel. Falling asleep at the wheel causes more than 20 per cent of accidents on dull, monotonous roads such 0…..as…. motorways, according 1 to a recent medical report.

Typically, these involve running 2 off the road or into the back of another vehicle, and are more likely to cause serious injury 3 because of the sleepy driver's failure to brake. Of course it is a fact that the body's biological clock has a major influence, as these accidents peak at times 4 when sleepiness is naturally higher.

Men under 30 are the 5 most vulnerable, as they are the drivers most typically out on the roads 6 in the early hours. Apparently, they take more risks in driving when sleepy (it must 7 be added, however, that alcohol is not a factor here).

In contrast, older people suffer from an early afternoon sleepiness, when they are 8 more at risk.

The time 9 of the day was found to be as important as the length of the drive, yet practical advice to drivers often concentrates more 10 on the length of the drive 11 than its timing.

Devices are now 12 being marketed as in-car monitors of driver sleepiness to warn drivers, but doubts 13 are being raised about these devices.

What is the advantage of alerting drivers already aware that they are sleepy but 14 who still persist in driving? What is more, these devices are of unproven reliability and may simply encourage drivers to take further risks.

The best advice is either to find 15 some reasons not to drive at night, or to get the car off the road as soon as possible, if you're not feeling too bright and breezy!