Driving impairment due to sleepiness is exacerbated by low alcohol intake

Abstract

Aims: To assess whether low blood alcohol concentrations (BACs), at around half the UK legal driving limit, and undetectable by police roadside breathalysers, further impair driving already affected by sleepiness, particularly in young men, who are the most "at risk" group of drivers for having sleep related crashes. Methods: Twelve healthy young men drove for two hours in the afternoon, in an instrumented car on a simulated motorway. In a repeated measures, counterbalanced design, they were given alcohol or placebo under conditions of normal sleep or prior sleep restriction. Measurements were: driving impairment (lane drifting), subjective sleepiness, and EEG measures of sleepiness. Results: Whereas sleep restriction and alcohol each caused a significant deterioration in all indices, the combined alcohol and sleep restriction further and significantly worsened lane drifting (which typifies sleep related crashes). This combined effect was also reflected to a significant extent in the EEG, but not with subjective sleepiness. That is, alcohol did not significantly increase subjective sleepiness in combination with sleep loss when compared with sleep loss alone. Conclusions: Modest, and apparently "safe" levels of alcohol intake exacerbate driving impairment due to sleepiness. The sleepy drivers seemed not to have realised that alcohol had increased their sleepiness to an extent that was clearly reflected by a greater driving impairment and in the EEG. Driver sleepiness causes about 10% of all UK road crashes, with a higher rate for monotonous motorways and similar trunk roads.1 2 Such crashes are more likely to result in death or serious injury 2 owing to a failure to brake beforehand and a higher impact speed. A recent epidemiological study from France 3 indicated that this outcome is more likely if sleepiness is combined with alcohol, even with blood alcohol concentrations (BACs) as low as 0.010 g alcohol/100 ml blood (0.01%). The UK alcohol limit for drivers is 0.08% (equivalent to a breath alcohol concentration of 35 µg alcohol/ 100 ml breath). Other EU countries have similar or lower legal limits, and in the USA it is generally 0.10% for drivers over 21 years, but 0.02% for those under 21 years.Three driving studies have combined alcohol with sleepiness.4-6 All used: only simple (computer console only) driving simulators, relatively high BACs (0.05-0.08%), either minimal 6 or marked 4 5 sleep loss, and only short driving periods. One 6 was restricted to women (there are sex differences in alcohol elimination 7 ), and was concerned with circadian effects of alcohol rather than with sleepiness itself. There is the opportunity for a more realistic study involving: a full size car simulator incorporating lengthy and monotonous driving, low BACs well within the "pass" region (BACs <0.06%) of police roadside breathalysers, and a commonly found level of sleepiness, typified in the afternoon and following a night of curtailed sleep. As well as having these features, our study focused on young men, as most (90%) sleep...

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