Young male drivers feature predominately in road crash statistics, especially where the driver has fallen asleep. One possibility is that they are more vulnerable to the effects of sleep loss compared with older men. We assessed the effect of normal night sleep versus prior sleep restricted to 5h, in a counterbalanced design, on prolonged afternoon driving in 20 younger (av. 23y) and 20 older (av. 67y) healthy men who drove a full size, real car simulator under monotonous ‘motorway’ conditions for 2h during the ‘afternoon dip’. Driving was monitored for sleepiness related lane deviations, EEGs were recorded continuously and subjective ratings of sleepiness taken every 200sec throughout the drive. Following normal sleep there were no differences between groups for any measure. After sleep restriction, and compared with the older group, the younger drivers showed significantly more sleepiness-related deviations, greater 4-11Hz EEG power, and a near significant increase of subjective sleepiness. Correlations between the EEG and subjective measures were highly significant for both groups, indicating good self-insight into increasing sleepiness. This study confirms the greater vulnerability of younger drivers to sleep loss under prolonged driving, even during the early afternoon.