There is no clear definition of fatigue. The term is attributed with various meanings: physical (e.g. as a result of heavy and protracted physical labour) and neurobiological (biologically determined sleep-awake rhythms) (Nordbakke et al, 2007). In addition, it also has a mental/psychological meaning: not having the energy to do something, and a subjectively experienced reluctance to continue with a task. Fatigue, or tiredness, concerns the inability or disinclination to continue an activity, generally because the activity has been going on for “too long". There are different kinds, such as local physical fatigue (e.g. in a skeletal or ocular muscle), general physical fatigue (following heavy manual labour) or “central nervous” fatigue (sleepiness). The last of these is mental fatigue – not “having the energy” to do anything. Sleepiness is a particularly important form of fatigue related to the level of brain stimulation and the structures that regulate it (Åkerstedt and Kecklund, 2000).