Early evening low alcohol intake also worsens sleepiness-related driving 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.