Prolonged activity inevitably leads to physical and mental fatigue. Researchers have related the duration of activity, or the so called time-on-task, to fatigue symptoms. One of the causes of driver fatigue is the time-on-task, i.e. the time spent driving. The fatigue-inducing effects of prolonged driving may be decreased by taking frequent breaks . For professional drivers, the relevant time-on-task is better seen as the total work time (including the time of driving). Professional drivers often perform many more tasks than the job of driving. For professional drivers, long working hours often go together with early waking and reduced sleep.