There is a circadian propensity for sleepiness in the early afternoon (contrasting with an alertness peak early evening) which potentiates the sedating effects of alcohol. However, little research has been done to examine these effects on driving performance. Four units of alcohol (95 ml of 40% spirit) or placebo were given double blind, with a snack to two groups ('early afternoon' and 'early evening') of 12 young women, at either 1310 h or 1810 h. Blood alcohol levels (BACs) were estimated by breathalyser. BACs were within the UK legal driving maximum. Subjects underwent 40 min of monotonous motorway driving in a car simulator. Self-ratings on sleepiness and alcohol effects showed a significantly greater impact of alcohol in the early afternoon relative to the early evening. Whilst neither time of day nor alcohol affected lateral corrective steering movements, alcohol significantly increased the average following-distance and the variability in this distance, especially during the early afternoon. In some subjects from the early afternoon group, this impairment seemed to be to dangerous levels following alcohol.