Dead tired or dead drunk? Fatigue kills - The results

You won't be surprised to learn that drink seriously affects your driving ability.

By 10.10pm, Peter had consumed six units of alcohol. A test confirmed he had 101 milligrammes of alcohol per 100 millilitres of blood. The legal limit is 80 milligrammes, so Peter was about 25% over the limit.

His ability to hold a steady distance from the car in front fell by 25%, and he was 23% worse at keeping to the centre of his lane.

At this time of night, David's driving wasn't badly affected, but it was a different story after no sleep at 2am.

Compared with his best result, David's ability to keep a steady distance from the car in front was 39% worse, and his driving within the lane had deteriorated by 37%. His reaction time was 53% slower than it had been earlier.

Professor Parkes is keen to emphasise that you can't read too much into a study of two people, and that a rigorous full-scale trial would involve at least 30 tired drivers and 30 drunks.

Even so, it's a shock to discover that a sober person's driving at 2am could have suffered more than a drunk driver's after six units. The designated driver at the office night out may not be as safe as you think.