Sleepiness combined with low alcohol intake in women drivers: greater impairment but better perception than men?

Objectives: We have previously shown that low blood alcohol concentrations (BAC) (at approximately half the legal driving limit in both the United Kingdom and in most states in the United States) exacerbate moderate sleepiness (sleep during the night restricted to 5 hours) and markedly impair driving ability in young men. There are distinct physiologic sex differences in the absorption, metabolism, and central nervous system effects of alcohol; therefore, we replicated this earlier study, this time using women and using similar BAC to provide a comparison.

Design: 2 x 2 repeated-measures counterbalanced.

Setting: 2-hour drive from 2:00 pm in an instrumented car on a simulated highway.

Interventions: Alcohol versus control and normal sleep versus sleep restricted to 5 hours.

Measurements And Results: Driving impairment (lane drifting), subjective sleepiness, and electroencephalographic measures of sleepiness. Sleep restriction significantly worsened driving performance and subjective sleepiness as it had in men. Surprisingly, unlike men, women showed no apparent adverse effects of alcohol alone on these indexes; they seemingly compensated for the effects of alcohol. However, alcohol's effects were profound when alcohol was combined with sleep restriction; nevertheless, women, unlike men, were aware of this enhanced sleepiness. After alcohol ingestion, the electroencephalogram showed increased beta activity, an effect not seen in men, indicating a differential pharmacokinetic effect of alcohol on the central nervous system, compensatory effort, or both. Debriefing questionnaires indicated that women were aware of the varying risks of driving under these different conditions.

Conclusions: 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.