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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Legally "safe" BAC markedly worsen sleepiness-impaired driving in women. However, they seem to be aware of their impaired driving and are able to judge the degree of risk entailed. Such an attitude may contribute to the lower incidence of sleep- or alcohol-related crashes in women compared with men.

Epidemiological findings point to very low blood alcohol levels heightening the risk of sleep-related fatal road crashes. This was further assessed using a full sized interactive car simulator. Twenty, sleep restricted, healthy young men underwent a 2 h simulated afternoon monotonous drive, having previously consumed nil alcohol or 3 units >90 min previously, and having near-zero breath alcohol (BrACs) at the start of the drive. In a repeated measures, double-blind, balanced design, driving performance, subjective sleepiness and EEG were monitored throughout. Compared with nil alcohol, the alcohol condition initially increased sleepiness-related driving impairment. However, this was not mirrored by subjective sleepiness or EEG. An unexpected reversal (i.e. improvement) in driving impairment occurred with the alcohol group, in the second hour of the drive. This was supported by a trend for improved subjective alertness. Alcohol continued to interact with sleepiness-related driving impairment after BrACs had reached zero. However, a lack of subjective perception of increased sleepiness, at this time, further points to the dangerous combination of even modest alcohol intake and sleepiness, and confirms the road crash findings. BrACs are a poor guide to driver impairment.

Following night-time sleep restriction, afternoon driving performance during the bi-circadian surge in afternoon sleepiness is markedly worsened by blood alcohol concentrations (BACs) well under most national driving limits. This study assessed how driving with this same sleep restriction and BACs (av 40 mg and 28 mg alcohol/100 ml blood at the beginning and end of drive, respectively) respond during the evening circadian rise in alertness. In a 2 x 2 (alcohol versus control drink [double blind] x normal night sleep versus sleep restricted), repeated-measures design, eight healthy young men drove for 2 h from 18:00 h, in a real-car simulator, on a monotonous, simulated highway. Driving impairment (lane drifting), subjective sleepiness and EEG measures of sleepiness were recorded. While sleep restriction alone produced significant impairments to evening driving and subjective sleepiness, alcohol alone did not. However, alcohol combined with sleep restriction significantly worsened all indices, although, this was less than that found for afternoon driving with identical interventions. Whereas low BACs may not affect driving in normally alert drivers in the early evening, the addition of moderate sleep restriction still produces a dangerous combination. Probably, there is no 'safe' level of alcohol intake for otherwise sleepy drivers, at any time of the day.