Biological rhythm and womens's night-shift work : an occupational and physiological study of effects and mechanisms.

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This study attempted 1) to establish a new method of time-series analysis for nonlinear physiological data , 2) to clarify the relationships between environmental, occupational and psycho-social factors of women's night-shift work utilizing a behavioral-medical approach, and 3) to elucidate the effects and their mechanisms on biological rhythm in the cardio-vascular system. Maximum entropy power spectral analysis and the least square method were performed to extract the periodic structure and the best fitting curves from physiological time-series data including blood pressure and heart rate recorded longer than 24 hours. As the residuals and their standard deviations between the raw data and the best fitting curves varied from only 10^<-8> to 10^<-9>, this method of analysis using artificial periodic functions is very effective to quantitatively evaluate changes of biological rhythm. Subjective fatigue symptoms at the end of work, and each phase (6 or 7 hours) of periods extracted from time-series data of blood pressure measured at 15 or 30 minutes interval, were similar for day- and nightshift workers. However, in the daytime after continuous night work, sleep with intermediate arousal accompanied by an increase in blood pressure and heart rate were observed. Psychological factors referred to as daily stress in addition to insufficient sleep may possibly have affected the biological rhythm. From the above, it is suggested that sleep in night-shift work and the reduction of daily stress are effective to maintain biological rhythm. Further, although the sexual differences of changes in biological rhythm by night-shift work were not clarified, the importance of health administration in the occupational fields is suggested, particulary in the recent process of the Ministry of Labor giving access to women to night-shift work.