Even drinking small amounts of alcohol at lunchtime could turn you into a hazard on the roads in the afternoon, research has shown.
Even a modest amount of alcohol could be dangerous
Volunteers were well under the UK drink-drive limit - but the combination of afternoon tiredness and alcohol led to serious mistakes in a driving simulator.
The test results add fuel to campaigns for a cut in the legal limit, which is among the highest in Europe.
The volunteers were often quite unaware that their driving skills were seriously impaired by just one drink.
It is thought that driver sleepiness causes about one in 10 crashes - and probably double that figure on "boring" roads such as motorways.
The body's natural rhythms mean that most people have a dip in alertness in the middle of the afternoon.
Scientists from the Sleep Research Centre at Loughborough University wanted to see whether the combination of even small quantities of alcohol exacerbated this problem.
They gave healthy men a large vodka and orange alongside two cheese rolls - the equivalent of a pub lunch.
Some had been given a good night's sleep the night before, and some had been given a night of broken sleep to simulate existing tiredness.
All were breathalysed, and gave results roughly equivalent to half the UK drink drive alcohol limit of 80 milligrams per 100 millilitres of blood.
Then they were asked to "drive" a car simulation set up to mimic a long stretch of motorway, and their performance compared with non-drinkers and those simply deprived of a little sleep.
The results showed losing sleep was not good for driving skills - drivers occasionally "drifted" between lanes as their attention slackened.
However, the combination of a little alcohol and tiredness was markedly worse, even hours after the drink had been consumed.
Volunteers who had slept well and then drank alcohol at lunchtime also had impaired performance.
Brain activity monitoring backed up these findings.
The study, published in the Journal of Occupational and Environmental Medicine, said: "Because of the natural afternoon 'dip' in alertness, even after a normal night's sleep, a modest alcohol intake at lunchtime presents a potential danger to driving at this time."
Andrew McNeil, director of the Institute of Alcohol Studies, which has long campaigned for a cut in the legal alcohol driving limit, said the findings were not surprising.
He said: "This government came into power promising to cut the drink drive limit - but has reneged on that.
Volunteers were under the legal limit
"This is not the kind of effect that an ordinary person would notice if they were walking down the road - it comes into play when they have to respond to a sudden incident."
Roger Vincent, from the Royal Society for Prevention of Accidents, told BBC News Online: "We have known for some time that there is a problem with alcohol in combination with the early part of the morning or the afternoon.
"Just because it's a legal limit doesn't mean it's a safe one."