Progression of fatigue
The progression of fatigue research has been studied with vigilance tasks. A vigilance task is a type of task where a user must maintain attention on the task while waiting for and responding to an uncommon, unpredictable event, such as monitoring security cameras or a radar display. With the use of vigilance tasks, fatigue research has shown that periods of normal performance (i.e. seeing signals on time and providing right response) alternate with short lapses in functioning (i.e. missing signals or responding very late) . A theoretical explanation is that fatigue is not simply a passive process. Fatigue is the result of an interaction between deactivation processes (e.g. slower functioning; lesser attention) and compensation processes. This means that a person can react actively when he or she notices the onset of fatigue, and may compensate for increased fatigue, for instance, by putting in extra mental or physical effort to perform a task. The interaction of on-going fatigue and compensation (extra effort) leads to a performance that becomes increasingly variable or unstable. Thus, there is not a simple monotonous decrease of performance. Instead there is an increasing variability in performance, with more and faster changes between normal functioning and erratic functioning .