The Interactive Effects of Extended Wakefulness and Low-dose Alcohol on Simulated Driving and Vigilance
ROAD-SAFETY CAMPAIGNS IN AUSTRALIA AND THROUGHOUT THE WORLD AIMED AT REDUCING ALCOHOL-RELATED TRAFFIC DEATHS HAVE INCREASED public awareness of the risks of drinking and driving. Alcohol has serious deleterious effects on driving performance and perceptual judgement. A review of 200 scientific papers found that reaction-time, divided-attention, and psychomotor-function impairments are evident at blood alcohol concentrations (BAC) from 0.1% down to as low as 0.03%.1 Alcohol concentrations above 0.05%, but below 0.08%, appreciably increase the risk of having a crash.1,2 Based on this evidence, the BAC level of 0.05% has been set as the legal limit for driving in Victoria and in all other states and territories of Australia. Sleepiness also increases the risk of road crashes.3 It is estimated that 15% to 30% of traffic accidents are directly related to driver sleepiness, as determined by crash circumstances.4 Although there is a clear association between BAC and both simulated driving performance and road crashes,5,6 no laboratory-based objective measure of sleepiness has been shown to relate to crash risk. Comparison of performance deterioration due to sleepiness with performance changes caused by alcohol (BAC of 0.05% or above) is one means of determining whether a particular level of sleepiness is likely to increase accident risk. Performance on a variety of tasks after 25 hours awake is similar to performance at a BAC of 0.08% to 0.10% in a non–sleep-deprived state.7–9 These studies also found impaired performance at relatively modest BACs below 0.05%. Although there are conclusive data regarding the separate effects of alcohol and sleepiness on driving, in real-life situations it is common for these two conditions to occur simultaneously (alcohol-related accidents occur more commonly in the early hours of the morning).10 Data from roadside alcohol breath testing of drivers in Victoria, Australia, has revealed that the peak incidence of elevated alcohol levels occurs between 01:00 and 04:00. Nine percent of drivers tested on weekends during this time had a BAC between 0.010% and 0.049%, with 7.0% having a BAC of 0.050% or above.11 In contrast, only 1.4% of drivers had a BAC between 0.010% and 0.049% between 07:00 and 14:00. Thus, there are many drivers in whom alcohol ingestion may interact with the effects of extended wakefulness.11 A roadside survey in Finland found that the rate of low alcohol readings (BAC < 0.05%) among drivers has doubled in the past 8 years and is three times more frequent at night.12 The combination of alcohol and sleepiness appears to be synergistic, with the relative risk of death in crashes related to alcohol and fatigue combined being greater than the sum of the individual effects.13 A small number of experimental studies have assessed the combined effects of sleepiness and alcohol on simulated driving performance. Some studies have demonstrated a significant decline in driving performance when alcohol and sleep deprivation are combined, in comparison with either factor alone,14–17 whereas others have found no synergistic change in performance.18 Various methods of inducing sleepiness have been employed in these studies, including sleep restriction to 5 hours the previous night,14,16 circadian effects,15,19 and extended wakefulness.18 Varying alcohol levels have been used, including legal and illegal levels for driving. These studies raise the issue of when (due to circadian-rhythm effects or sleep deprivation) legal limits of alcohol may become a risk to the driver.14,17,20 However, a limitation of these studies is that the level of impairment observed is difficult to equate to a level known to increase crash risk. The current study assessed the effects of a legal BAC (0.03%) during extended wakefulness (also during the circadian nadir) on vigilance and simulated driving. Performance at a BAC of 0.03% during extended wakefulness was compared with performance at a BAC of 0.05% in a rested state. There are no published studies of this latter comparison, which is critical in that it may suggest that a combination of low blood alcohol levels and extended wakefulness could increase crash risk. In addition, this is the only study that has assessed the effect of extended wakefulness without prior sleep restriction in combination with low blood alcohol levels. It was hypothesized that driving performance and vigilance would be worse at 0.03% BAC after extended wakefulness, when compared with 0.05% BAC in a rested state.