By Julian Joyce BBC News

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.

This time it's not the after-effects of yet another Christmas party, it's about trying to understand the uncertainty behind this year's anti-drink driving campaign.

ABSORPTION OF ALCOHOL 20% absorbed in stomach/80% in upper small intestine Most rapid when stomach is empty Accelerated by tolerance (habituation) to alcohol Decreased by food in stomach Other factors include emotional state, drugs, type of alcohol drunk Source: Department of Forensic Medicine, University of Dundee

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?

According to government guidelines it takes at least one hour for every half pint of beer, glass of wine or measure of spirits to get out your system. You should count the hours from the time you finished your last drink.

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 Brake guidance

Most obviously, we are all different shapes and sizes, with different metabolisms - no two people are the same. Because of this there can be no definitive answer about when you are safe to drive after a heavy night - it's impossible to generalise.

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.

A home breath test gives guidance to drivers

I've clearly had too much to drive by the end of the evening, and I get the bus home. To prove my inebriation I test myself when I arrive back using an AlcoSense portable breathalyser.

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 the screen predictably flashes red and beeps loudly. 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. The reading is clear - if the machine is to be believed, over the course of the night the alcohol has dissipated from my system.

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

Flashing light

It's known as a Police-issue Electronic Screening Device (ESD) and works on the traffic-light system. Green means no or very little alcohol in the system, amber means you are close to the limit and red says you are over it.

Taking the official test, courtesy of the Met Police

"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.

Again 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."

The last sentence sums it up: playing the numbers game is playing with firewater. Don't drink enough to float a loan to Northern Rock, and then expect the same from your body as last time. You won't be able to plead in court that your body was inaccurate!
Nigel Macarthur, London, England

There really is only one way to be sure you're fit enough to drive the next day, and that's to not drink the night before - or at least limit yourself to one or two only. I never drink & drive, or drink heavily the night before if I know I have to drive the next morning. Its simply not worth it. Shame on those who do.
Tony, Croydon

Cars need to be fitted with breathalysers, breathe into them, and if you're below the limit, you can drive. It would stop 90% of drunk driving. The only way around is to have somebody else breathe into it- and make that an offence. I can't see why this hasn't been done yet.
Alistair , Caerphilly

This focus on morning blood alcohol must be taking police resources out of late night alcohol checks, although the report gives no indication of comparatively how many people are killed or seriously injured in morning alcohol-related accidents. I would have thought very few. While there are no excuses for drink driving (outside, perhaps, serious emergencies), limited resources should be deployed where most needed, and I wouldn't think the morning after is the place.
Jim Robertson, East Kilbride

That's amazing! I allow 2 hours per normal strength pint and 3 hours per premium pint. Thus If i'd finished 5 pints at 2345 I would count that as 15 units in me. The guideline is not to drive with more than 3 units so that would leave me unsafe to drive until mid-day the next day. I am amazed that you were clear at 7am. I'm a bit younger (36) A bit shorter (5'11"), a bit heavier and only cycle 6 miles per day. I wonder what difference that makes?
Tony Brett, Oxford

Another problem is that, just because the breath test says you aren't over the limit, you may not be fit to drive due to lack of sleep, dehydration headache etc. These things will distract you whilst driving and are dangerous too.
Mrs Mo, Manchester

After only about 1 pint of strong lager I can already begin feel the effects of drinking, and I would not drive, although I may well still be under the legal limit. However, the next morning, about 7 hours after finishing drinking about 12 pints, I don't feel drunk at all and I would feel safe to drive. At that time, I would probably be over the legal limit even though I didn't feel it. Its a complicated subject. According to the government advice, I should wait 24 hours after drinking 12 pints, so no driving at all the next day, but I don't know anyone who would wait that long.
Roger, Bristol, UK

Good article, but fails to emphasise one point: there is now absolutely no excuse for driving whilst over the limit. Obviously there may be exceptional cases in an emergency but I for one am sick of excuses by drinkers and references to the "nanny state". Perhaps those who make these comments could be shown the aftermath of a fatal accident caused by drink.
Neil Small, Scotland

If you can't be sure whether you are over the 80mg/ml limit, what hope have you got knowing whether or not you are over the zero limit that some people advocate?
Roger Smith, Shefford UK

As so many people are apparently driving over the limit the morning after, you would be forgiven for expecting absolute carnage on the roads during your morning commute in the run up to Christmas. Given the absence of any government claims about drink related accidents specifically occurring in the morning and day time, I suspect that the evidence fails to show a conclusive link between morning after blood alcohol and accident rates.
Darren, Cardiff

When I was a police officer I often arrested drivers who "had been careful" in their drinking. They did not realise that there are so many variables that the only safe policy is not to drink and drive. They got disqualified anyway. NO sympathy, I also went to crashes caused by drivers who had "been careful" in which people died. AS for the morning after, walk to work.
Barry P, Havant England