Background and Aim: According to the biorhythm theory when the phase shift from positive to negative and vice versa people experience a critical an unstable day that prone them to error and accident. The purpose of this study is to determine this relationship in one of the automobile manufacturing industry.

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Materials and Methods: At first 1280 person incident entered the study was reviewed and then the critical days of each biological cycle was determined using the software Easy Biorhythm Calculator for all persons.

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Results: This study showed that the frequency of accidents in critical days and negative section of physical cycle was more than expected. Also the frequency of accidents in critical days and negative section of emotional and intellectual cycles was less than expected.

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Conclusion: This study showed that the frequency of accidents in critical days and negative section of physical cycle was more than expected. Also the frequency of accidents in critical days and negative section of emotional and intellectual cycle was less than expected. Due to the physical nature of the work activities in the automobile manufacturing industry can be stated that the study showed that in physical work activities, frequency of accidents in critical days and negative section of physical cycle in which the person is not physically ready to do the job was more than expected.