The effects of sleep deprivation and legal alcohol intoxication on simulated driving performance

Several studies have shown the potential for negative synergistic effects of alcohol and sleepiness on driving, which is supported by studies of accident data. We have previously showed that sleep restricted young drivers had impaired driving simulator performance during the early morning (approx 0100h), translating to effects on speed deviation, steering deviation and number of crashes following low-dose alcohol, without significant changes in perception of sleepiness and driving performance. The present study investigated the effects of combining sleep restriction and alcohol (at two low doses, both resulting in legal BACs) on simulated driving performance during the mid-afternoon (approximately 1400h). The study used a repeated measures design with four experimental conditions. Normal sleep with no alcohol (CONTROL), sleep restriction (SR) alone and SR plus two low-dose BACs (SR + A1 ∼0.025 and SR + A2 ∼0.035g/dL). 21 healthy male volunteers, aged 22.3y (±3.7), free of medication and with normal BMI: 25.2 (±6.8) performed a 70-minute driving simulation task with measurements taken for steering deviation, braking reaction time and collisions. BACs were 0.0g/dL during CONTROL and SR conditions. Prior to the driving task during SR + A1 and SR + A2 conditions mean BAC was 0.025g/dL (±0.002) and 0.034g/dL (±0.002) respectively. After the drive, corresponding BACs were 0.013g/dL (±0.002) and 0.023 ± 0.001g/dL. Preliminary analyses of driving performance parameters suggest that steering deviation increased significantly, both over time (p < 0.01), and between conditions (p < 0.01), with no interactive effects. The highest deviations were observed during the SR + A2 condition. After preliminary analysis, braking reaction times in the alcohol conditions were delayed compared to control (p < 0.05). Collision data requires further analysis but it was evident that 43% of subjects had an off road crash and these occurred with increasing frequency when sleep restriction was combined with alcohol. These data suggest that the combination of sleep restriction and lowdose alcohol has a delayed, negative dose-dependent effect on simulated driving performance. Even at these low levels of alcohol intoxication – which would be free of legal implications, there is a measurable effect on steering, reaction time and accident likelihood. In addition, these effects appear to be exaggerated with time on task.