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Alcohol and driving under the limit

Alcohol combined with a lack of sleep dramatically increases your chances of having an accident whilst driving even below the legal drink-drive limit.

We all know that drugs and alcohol don’t mix. I even notice a difference when I take hay fever tablets. It may seem obvious that having alcohol in your system when you are tired will increase your impairment levels but the results will still surprise you.

Sleep deprivation is a factor in a quarter of all UK vehicle accidents so combining this with alcohol instantly increases your odds of being a part of that statistic. A study was done in 2003 measuring signs of impairment caused by a driver’s lack of sleep, alcohol consumption and both combined.

The first interesting result was that the test subjects’ alcohol consumption combined with regular sleep showed a lower BAC than when they drunk alcohol on limited sleep. In both tests the subjects’ consumed the same amount of food and alcohol so this backs up the statement that lack of sleep lowers your tolerance for alcohol.

Here is the outcome of their results showing the related breath alcohol concentrations for each test:

Alcohol and driving under the limit
Alcohol tolerance levels effected by lack of sleep






The current drink-driving limit in force in England and Wales is 80 mg alcohol / 100ml blood. The limit in Scotland was reduced to 50mg/100ml blood in 2014. So it is worth noting that all drivers that were involved in the above test and had alcohol in their system were in fact still legal to drive in Scotland.

The results shown in the table below reflect the number of incidents, e.g. lane drifting, that occurred.

Sleep and alcohol effects with driving
How alcohol and lack of sleep effect your driving abilities



The baseline shows the results for the drivers who have enjoyed a regular sleep and no alcohol. The same drivers were subsequently tested in the alternative three types of conditions on separate weeks.



The results for lack of sleep alone had a bigger impairment factor compared with alcohol consumption combined with regular sleep for the majority of the journey. However, both cases are clearly more dangerous situations for the driver than the baseline results.

The results for alcohol and lack of sleep combined dramatically increased the impairment level of the driver after the first 30 minutes.

Other tests done during this experiment looked at the level of sleepiness of the driver. Standard EEG power is indicative of sleepiness. The results below show that the addition of alcohol made the drivers more tired.

These results clearly support the fact that alcohol is a sedative and will impact the body and our ability to function effectively. Alcohol should always be consumed with caution.