In Driver Fatigue is the Number One Safety Issue in the Truck and Bus Industry, fatigue expert witness Dennis Wylie writes:
Fatigue causes cognitive impairments that affect vigilance, attention, perception, and decision making – processes that are crucial to safe driving. These objective, measurable cognitive fatigue effects can occur without any marked degree of prior physical exertion, and the driver may not be aware of these impairments. Another serious fatigue threat to safe driving is drowsiness – the tendency to fall asleep at the wheel. This is also a condition that drivers tend to underestimate (or to be totally unaware of) while it is actually happening to them. Circadian rhythm interacts powerfully with fatigue and sleep debt, and has a major impact on human error.
Driver fatigue is also partly a subjective experience (that is, one that the driver is aware of), characterized by lack of motivation, feelings of exhaustion, boredom, discomfort, and a disinclination to continue driving. These effects of fatigue can impair driving safety by impairing sustained attention and safe decision making.