The study focus on the incidence and importance of tiredness among young male car drivers in nighttime traffic. It took place on a major highway in Copenhagen County, Denmark, and was performed as roadside surveys in the hours between midnight and 6 A.M. The study consisted of a questionnaire with self-assessment of tiredness followed by a simple visual reaction-time test. Two hundred and forty nighttime drivers and 40 early morning drivers joined the study. One hundred and twenty-three nighttime drivers declared themselves as rested, 106 as tired, and 11 as very tired. Thirty-seven early morning drivers declared themselves as rested, while the remaining three declared themselves as tired. The drivers' self-assessed level of tiredness corresponded well with the results of the simple visual reaction-time test, but one self-declared tired driver had to be reclassified as very tired. The visual reaction-time test henceforth showed a mean reaction time of 0.189 seconds for the rested drivers, 0.223 seconds for the tired, and 0.309 seconds for the very tired nighttime drivers (p < .001). Mean reaction time for the early morning drivers was 0.190 seconds corresponding to 0.246 seconds in the equivalent nighttime group (p < .001). The fluctuations in mean reaction time throughout the night for the whole population including the early morning drivers correlated well with the number of tired/very tired drivers (corr. = 0.96). The study shows tiredness as a common affliction among young male nighttime drivers.