The most general factors that cause fatigue are lack of sleep, bad quality sleep and sleep
demands induced by the internal body clock. Besides these general factors, prolonged driving
(time-on-task) can increase driver fatigue, especially when drivers do not take sufficient
breaks. For specific groups of drivers, e.g. professional drivers, these general factors often
play a more persistent role due to long or irregular work schedules. A small part of the general
population (3-5%) has to cope with obstructive sleep apnoea, a sleeping disorder which
contributes to above average day-to-day sleepiness.