Nobody is going to arrest you for enjoying a big night out and we all doubt you would try and drive a car after drinking, but how often would you drive the next day? Many people would chose to go on a night out and stay at a friend's house before driving home or to work in the morning but this can be just as dangerous as driving the night before.
Depending on what country you are in, the legal limit for driving is around 50 - 80 milligrams of alcohol in 100 millilitres of blood. You will easily reach this limit with just a couple of units of alcohol so imagine how 10 - 20 units would effect you even after a nights sleep. Obviously the tolerance to alcohol depends on an individual but the general rule of thumb is that one unit remains in the bloodstream for one hour.
The build up of alcohol can often take more than 12 hours to process through the body and it can still have an effect up to a day later. Many people will often stay up to the early hours of the morning on a night out or at a party meaning they don't get a full nights sleep, especially if they are getting up at their usual waking hour. They assume that after sleeping the effects of the alcohol will have worn off but this is not the case.
A surprising number of people drive in the morning after drinking the night before, most do not realise that they are over the limit but some are not so innocent, they assume that even though they might still be breaking the law there is a much lower chance of being caught in the morning than at night when the police are waiting.
Most police departments will be aware of nights of heavy drinking such as weekends and holidays and will plan accordingly, the morning after Christmas, new years day and other holidays are times when forces set up road blocks and randomly stop drivers to check for alcohol content. This is primarily a deterrent but they also aim to crack down on offenders.
It's not just the effect of drinking that can impair driving ability, a heavy hangover means the brain is dehydrated so you will be less alert, lethargic and have impaired vision, not to mention a sore head. Combined with residual alcohol levels this makes for a deadly combination that can catch unsuspecting victims with devastating consequences. The simple answer is to wait at least 12 hours and if you feel hung over, don't drive.