One large vodka and orange at lunchtime is enough to impair driving skills and reduce mental alertness even though the driver is still below the legal blood alcohol limit, researchers said on Thursday.

The effect was even more apparent when a driver had had a poor night’s sleep, according to Prof. Jim Horne, of the sleep research centre at Loughborough University.

The combination of the low level of alcohol and the naturally occurring “afternoon dip” in mental alertness affected driving performance significantly, Prof. Horne reports in a study published on Thursday.

Although the driver’s brainwaves registered the lack of alertness, the drivers were unaware of their condition.

Driving impairment, measured by “lane drifting”, was 10 times more frequent 30 to 60 minutes after the lunchtime drink in the men who had slept poorly than in those who had drunk no alcohol and slept well. The effect was less immediately after drinking but still five times greater two hours later.

“We are talking about micro-sleeps when the driver sleeps for a few seconds, the eyes start drooping and the expression is glazed.

“The effect is that you do not make the normal corrections and when you are whizzing along at speed this becomes significant,” Prof. Horne said yesterday.

“People don’t seem to realise that alcohol, even in small amounts, can be dangerous and that alcohol is soporific in its own right.

“It is the same when people who go to parties and drink very little and then are abstinent and drive home at 2 am or 3 am. They may be below the limit but they don’t realise they may be affected. People should be aware of this.”

The study, published in the journal Occupational and Environmental Medicine, says sleepiness accounts for about 10 per cent of all road crashes, with the risk increased on motorways and other monotonous roads.

These accidents result in more serious injuries and death because of the driver’s failure to brake and the higher impact of the crash.

The study follows French research which showed that when sleepiness was combined with alcohol, even in very small amounts, accidents were more likely.

The French researchers found an effect when the driver had only 0.01 grams of alcohol per 100 ml of blood. In Britain the alcohol limit for drivers is 0.08 g/100 ml.