Towards a New Framework of the Study Method of Workers' Fatigue : Attaching Importance to the Recovering Process of Fatigue

Fatigue is a physiological phenomenon which has three processes, i. e., it is generated and developed by doing an activity such as work, learning or sport, and is recovered by rest or sleep taken after the activity. However, previous studies on workers' fatigue in Japan have inclined to make clear the former two fatigue processes of generation and development, resulting in neglect of the third process of recovery. In the present paper the author stressed that studies on workers' fatigue in Japan must attach importance to its recovering process for future improvement of the studies. The paper consists of three parts. In the first part the author criticized the study method of workers' fatigue prevailing in Japan which was proposed by the Industrial Fatigue Research Committee of the Japan Association of Industrial Health in the 1950 s. Based on the criticism, the author proposed a new concept of workers' fatigue, and indicated two specific issues to be considered in study methods of workers' fatigue, which human biological fatigue studies do not involve. The first issue is that workers' fatigue is of special significance in its effect upon their private, family or social lives which will be done after work. And the second one is that such study method should include fatigue assessment on whether workers examined in a fatigue survey are over-fatigued or not. The author insisted that an essential criterion for the fatigue assessment should be Recoverability, i. e., that human fatigue can be recovered by sleep every night or by rest and sleep on weekend holidays. Thus an over-fatigued condition is defined as contrary to the criterion of recoverability. The second part of the present paper contained how to make fatigue assessment from a viewpoint of the recovery of fatigue, based on results of two field surveys. In the first survey on interview it was examined whether 17 day workers attacked by fatal cardiovascular or cerebrovascular injuries had been over-fatigued in the 3-month period prior to the attack. The results of the survey showed that two thirds of the attacked workers had unusually behaved showing strong demands for sleep in order to recover from their fatigue due to work. It was suggested that these workers had been explicitly over-fatigued. Further, one third of the attacked workers had demonstrated behavior showing the lack of energy, often accompanied with complaints of sleep shortage at home. These workers had obviously been in a state of extreme exhaustion. The second survey concerned a fatigue assessment on 25 electronics workers working in a permanent night work system. The work ?system consists of three consecutive 12 hr night shifts without any nap followed by four consecutive days-off. The results of the survey showed that on the straight off-duty days, the workers tended to have a longer night sleep as the first night shift approached, the longest on the last day. Time for both home and outdoor activities of the workers was markedly shortened in the daytime between two night shifts. These results suggested that the permanent night system is disadvantageous hampering the workers' daily activity needs. In the third part of the paper included were results of two experimental studies on a relationship between two fatigue processes of development and recovery. In these experiments, the development of fatigue was defined as a decrease in cerebral arousal level during wakefulness, which was manifested by high scores of the Stanford Sleepiness Scale, those of the Fatigue Feeling Scale which was proposed by the Industrial Fatigue Research Committee of the Japan Society for Occupational Health, or decreases of Critical Fusion Frequencies. The recovery of fatigue was considered as an increase in duration of stage 4 sleep (S4) or Slow Wave Sleep (SWS) in sleep taken during a sleeping period. The first experiment examined relations between the two processes under the condition in which a cycle of 22 hr-wake and 8hr-sleep was repeated four times. In the second experiment, the effects of three nocturnal nap conditions of no nap, a 1hr nap and a 2 hr nap on the cerebral arousal level during subsequent morning hours were compared. The results of the two experiments showed that a decrease in the cerebral arousal level during wakefulness and an increase in duration of S4 or SWS in sleep were not related to each other in prior wakefulness of approximately 20 hr