Fatigue affects mood and motivation as well as psychomotoric and cognitive functions . Fatigue is partly a subjective experience, characterized by lack of motivation, feelings of exhaustion, boredom, discomfort, and a disinclination to continue the task at hand. At the cognitive level, studies have linked sleepiness and fatigue to decreases in vigilance (capacity to detect and respond to unpredictable signals or events over a longer period of time), reaction time, memory, psychomotor coordination, information processing and decision making . Its effects are strongest in those tasks that are monotonous, that have long duration that demand constant attention and that have low predictability.
The part of fatigue which is psychological in nature has also been called 'mental fatigue' . Mental fatigue is a gradual and cumulative process and is associated with unwillingness to put in effort, reduced efficiency and alertness and impaired mental performance (Grandjean, 1988 as cited in: Lal & Craig, ). According to Grandjean (1979 as cited in: Lal & Craig ), the functional states of a person range from deep sleep, light sleep, drowsy, weary, hardly awake, relaxed, resting, fresh, alert, very alert, stimulated and a state of alarm. In this series, mental fatigue is a functional state, which may result either into sleep or into a relaxed, restful condition.