What is fatigue?
In the literature many definitions are used for fatigue. The concepts of "fatigue", "sleepiness" and "drowsiness" are often used interchangeably. Sleepiness is an aspect of fatigue which is perhaps easiest to define. Sleepiness can be defined as the neurobiological need to sleep [79], resulting from physiological wake and sleep drives [55]. Fatigue has from the beginning been associated with task performance. In addition, fatigue also has a psychological meaning: not having the energy to do anything, and a subjectively experienced reluctance to continue with a task [15]. Thus, sleepiness is the drive for sleep while fatigue can be seen as a signal from the body that we should end the ongoing activity, whether it is physical activity, mental activity or just being awake. Although the causes of fatigue and sleepiness may be different, the effects of sleepiness and fatigue are very much the same, namely a decrease in mental and physical performance capacity.
Below we describe physiological and psychological components of fatigue and pay attention to progression of fatigue