By Julian Joyce BBC News

This article is an abridged edited article reproduced with kind permission of The BBC.

It's 8 o'clock in the morning after the night before. There's a background ache in my head, the product of too many strong lagers. My mouth is dry and there's a vague uneasiness in my stomach.

It warns that people getting behind a wheel the morning after a big night out - while they are still over the legal alcohol limit or unfit to drive - is a growing danger.

Statistics back up the message. One in five drivers found drink-driving are caught the morning after, according to the Department for Transport (DfT).

More than a third of us underestimate when it's safe to drive the next day and nearly half of young drivers admit driving after drinking heavily the previous evening, according to figures from the road safety charity Brake.

And there are no shortcuts. The sobering qualities of a cold shower or a cup of coffee are just a myth; they won't make you fit to drive any faster and it's dangerous to think so, say experts.

Time is the only thing that will to get the alcohol out of your system and the big question is how much?

Impossible to know

But it's a confusing subject. Just this week, a survey found people in their 30s and 40s are more likely than 20-somethings to say they've drunk too much. One explanation mooted by Alcohol Concern is that, with age, alcohol stays in the blood system longer.

Then there's the varying glass sizes and the increased strength of alcoholic drinks. The Office for National Statistics announced last week it had revamped its assessment methods for how much alcohol we drink because of these two issues.

It all depends on body-size, what you have eaten and your general level of fitness

Nevertheless, it's a question we might reasonably ask ourselves. There are some qualifications needed at this point. I'm aware I should not try to apply my conclusions to the population as a whole, as campaigners point out.

"Alcohol stays in the body for different lengths of time, depending on a variety of factors," says a Brake spokeswoman. "It all depends on body-size, what you have eaten and your general level of fitness."

So here are the facts: I am above average build - 6ft 1in tall and in my mid 40s; I weigh 15 stone and 12 pounds and although I'm overweight I like to consider myself generally fit. I cycle about 12 miles to and from work each day and would normally describe myself as a "moderate" drinker.

The experts agree I would naturally be more affected by alcohol if I were smaller, or female.

Well over the limit

My night on the town isn't a bacchanal by many people's standards. I drank five pints of premium-strength over the course of the evening, while enjoying a two-course meal. I finished drinking at 2345 GMT.

It's specifically marketed at motorists who want to make sure they are sober enough to drive in the morning. The device looks not unlike a mobile phone with a tube attached. You blow into it and it tells you whether you are under, borderline or over the drink-drive limit.

When I get home at 0100 and blow into the device my blood alcohol concentration (BAC) reads 0.11%, well over the legal limit of 0.08%. This is probably near the peak of my alcohol absorption, according to experts.

At 0700 the following morning I get up and test myself again using the same device. To my surprise, the reading is 0.00%.

To make sure my results are genuine, I'd previously arranged to be tested by a police officer with an official breathalyser kit.

Police Test

"Blow into the tube, as if you are blowing up a balloon," says PC Chris Lavender of the Osterley Safer Neighbourhoods Team, who conducts the test.

The light flashes green. To make sure, I'm re-tested twice and the light is green both times.

It's a vindication of the message that everyone is different, says PC Lavender.

But even that is only half the story, says Dr David Sadler, from the Department of Forensic Medicine at Aberdeen University. On another day, I could have been over the limit.

Not only is everyone different, the same body can react in different ways on different occasions, says Dr Sadler.

"How inebriated people get comes down to the rate of alcohol absorption from the intestines and that's affected by so many things including food, drugs - even what mood you are in," he says.

"It's the same for eliminating alcohol, which is the issue with driving the morning after. The time it takes to sober up can be affected by so many things and often people simply don't give themselves enough time. The only fool-proof way to not drink and drive is simply to do one or the other."

Article Reproduced By Kind Permission of The BBC

This article is an abridged and edited version of the article that appeared on the BBC News website.