Conclusions

Fatigue has a physical and a mental aspect.
Fatigue is associated with both reduced capacity to perform and motivation to perform.
Although sleepiness and fatigue may have different causes, their effects on performance and motivation are similar, a decrease in mental and physical functioning.
When fatigued, persons may alternate normal functioning with short lapses in performance (i.e. not noticing or responding to signals). The long term result of fatigue is an increasing variability of performance.