Taken from Defayolle et al., 1971
DEFINITIONS
Alertness levels also called wakefulness or level of arousal, or central nervous system activation, represent a continuum from hyperexcitation to deep sleep. In other words, there is no difference in the nature of these levels, they only differ by their magnitude of activation. The notion of alertness level must be distinguished from that of performance level. The alertness level is only a component of the performance level. It constitutes a necessary but insufficient condition. The other condition being the voluntary investment by the operator, i.e. the maintenance of a certain attention or vigilance.
Each alertness level is associated with a probability of reaching a certain efficiency level. This means that being awake does not necessarily ensure an optimum efficiency level: too high a level (strong emotion) disorganizes attention, too low a level (drowsiness) does not allow attention to be fixed on any pertinent information.
An optimum efficiency level can be reached from wakefulness level called "attentive alertness".
Relaxation, dispersed alertness or alertness decrement, are not consciously perceived. Change of state is reversible: return to attentive alertness is possible following a modification in activity or environment. This alertness decrement state can be facilitated through monotony. In case of sleep pressure, this condition rapidly turns into drowsiness. In alertness decrement situations, the ability to detect rare or unexpected signals is impaired.
A nonlinear relation exists between wakefulness level and performance. This relation is shown on the following figure:
6.2. FATIGUE
The notion of fatigue must be distinguished from alertness decrement and from the need for sleep. In fact, fatigue may appear with a high alertness level.
It can be defined as "a set of manifestations generated by intense and prolonged work extending beyond a certain limit". These manifestations are accompanied by a "feeling of fatigue", and a less favorable relation between efficiency and the effort required to accomplish it. In other words, to maintain equivalent performance, the tired subject must make greater efforts. Pushed to the extreme, a very high workload performed during prolonged periods, interrupted by very short rests, may lead to the development of an exhaustion phenomenon. This phenomenon causes asthenic type pathological reactions.
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