In this section we examine some general aspects of fatigue and alertness. Then we discuss problems of underloading, which are especially common in control room work. Finally, we give a short example of the need for variety in the job and the risk of monotony due to lack of variation.
10.5.1 Fatigue and Alertness
Fatigue is a difficult concept to define and there are several different types (Astrand and Rodahl, 1977). Purely muscular fatigue is due to the build-up of lactic acid in the muscles that is experienced in more severe cases as pain in the muscles. In addition, the function and efficiency of the muscles is reduced, particularly the ability to perform ‘precision’ work. The risk of muscular fatigue is limited in modern control rooms, where the degree of muscular loading is usually low. The work can normally be carried out from a sitting posture, or in a combination of sitting and standing. Changes in the degree of alertness are another form of fatigue that occurs due to changes in the central nervous system.
One factor affecting the degree of alertness is the time of day. ‘Productivity’ or efficiency varies with the state of alertness, although different aspects of efficiency vary with time of day and degree of alertness. Normally, alertness is lowest early in the morning and highest later in the evening. There are, however, large individual variations here too. For some people, the times of lowest alertness is earlier, for others, later. For other people, the variation between the lowest and highest levels of alertness is relatively small. Figure 10.11 shows how efficiency and ability vary with the time of day. Another important factor that affects the state of alertness is the character of the task. This will be discussed in more detail later.
Figure 10.12 shows how performance ability varies with degree of alertness. This shows that there is an optimal degree of alertness, where the performance ability is at its best. If the degree of alertness is low, performance is also low; if, on the other hand, the level of alertness is very high, performance ability may also be low.
Direct to the cortex (primary information)
Tasks containing very little variation or jobs with very low levels of loading cause in turn a low level of alertness, and thereby reduced performance ability.
The alertness level can be affected by factors other than those connected with the job. In monotonous work with a low level of stimulation, alertness can be kept at a high level by means of a secondary stimulation, such as music. On the other hand, where the job provides a certain level of stimulation, and thereby a relatively good level of alertness, a secondary stimulation may result in too high a level of alertness and a reduction in efficiency. Music and other secondary stimuli while people work is thus only of value in tasks that are monotonous or where the mental load from the job is low.
Purely physiologically, the degree of alertness is affected by areas in the cortex of the brain. Signals from the various sense organs go first directly to their special areas in the cortex. In addition, signals go via a secondary route to the brain stem and bring about a general stimulation of the whole cortex. This general stimulation prepares the brain to receive signals and information from the various sense organs. If the secondary stimulation is too intense, there is a risk that it will not just stimulate but overstimulate and cause confusion in the interpretation of the primary information (see Figure 10.13).
Control room work is often characterised by low mental loading over long periods, thereby producing a low level of alertness. In addition, this type of work is often done on a shift basis, which means that people are often working during the early morning hours when alertness is naturally and physiologically low. The risk of poor performance is thus very great.