In industrialized countries one-fifth of all traffic accidents can be ascribed to sleepiness behind the wheel. Driver sleepiness can have many causes, including the use of medicinal drugs or prolonged driving. The present study compared the effects of prolonged highway driving at night with driving impairment caused by alcohol. A cross-over balanced design tested 14 healthy young men who drove three sessions during night-time on the open road. The driving sessions were of 2, 4 and 8 h (03:00–05:00, 01:00–05:00 and 21:00–05:00 hours) duration. Standard deviation of lateral position (SDLP, cm), measuring the weaving of the car in the last driving hour of each session, was the primary parameter. Only 2 h of continuous nocturnal driving were sufficient to produce driving impairment comparable to a blood alcohol concentration (BAC) of 0.05%; after 3 h of driving impairment corresponds to a BAC of 0.08%. In conclusion, a maximum of two continuous nocturnal driving hours should be recommended.