Sleepiness and perception during simulated driving: residual effects of alcohol?

We have previously shown that sleep-restricted young drivers had impaired driving simulator performance during the early morning(approx 0100h) following low-dose alcohol, without significant changes in subjective sleepiness or perception of driving performance.The present study investigated the effects of combining sleep restriction and alcohol (at two legal blood-alcohol concentrations) on subjective sleepiness and performance perception during the mid-afternoon(approx 1400h).The study used a repeated measures design, with four experimental conditions. Normal sleep with no alcohol (CONTROL), sleep restriction (0200–2066h = 4h: SR) alone and SR plus two low-dose blood alcohol concentrations (BAC) (SR + A1 ∼0.025 and SR + A2∼0.035g/dL). 21 healthy male volunteers, aged 22.3y (±3.7), with normal BMI 25.2 (±6.8), underwent a 70-minute driving simulation task with simultaneous EEG recording. Subjects also rated their subjective sleepiness and self-perception of driving performance under each condition. BACs were 0.0g/dL during the Control and SR conditions. Prior tothe 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, the corresponding BACs were 0.013g/dL (±0.002) and 0.023g/dL (±0.001). EEG alpha and theta power was higher in the SR +A2 condition compared to control. There were significant condition(p < 0.01), time (p < 0.01) and condition × time interaction (p < 0.01)effects on drowsiness and subjective driving performance ratings, with progressively poorer ratings in SR, SR + A1 and SR + A2 conditions as time on task increased, which appeared to partially recover after 40 minutes on task in SR, and SR + A1 conditions but not SR + A2. Compared with SR, the SR + A2 condition showed no increase in sleepiness or perceived driving impairment early in the drive (when BAC relatively high) but marked increase late in the drive (BAC low).These data suggest that the combination of sleep restriction and low dose alcohol has a delayed, negative dose-dependent effect on objective sleepiness. The increased drowsiness and driving performance decrements are well perceived by subjects even after highest alcohol during afternoon driving. There may be some evidence to support previous findings which suggest that alcohol continues to affect sleepiness related driving impairment when BAC has almost returned to zero, since both subjective and objective sleepiness were shown to increase while BACs were falling. Of particular interest, after 70 minutes of driving, there was clear separation between the A1 and A2 conditions (at only 0.015 and 0.025g/dL respectively).