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Introduction: In modern societies, shift work is an essential component of daily life. Since the beginning of research in shift work, researchers have been engaged in the identification of factors predicting adjustment and adaptation to shift work. The concept of adaptation and tolerance refers mainly to the biological response in terms of circadian rhythms and performance efficiency as well as sleep duration and quality. Since some of the shift workers show higher level of adaptation in comparison with the others, the individual sensitivity may be a good predictor. The present study was undertaken to specify whether changes in cortisol rythmicity are associated with chronotype, quality of sleep and adaptation to shift work. Methods: This study was cross-sectional. Blood samples were drawn from 57 shift workers. Three sample were drawn from each subject (at the beginning of their night shift, at the end of their night shift and at the beginning of their morning shift). The samples were immediately centrifuged and the plasma was removed and stored at -25 0c until assay. Cortisol was measured by radioimmunoassay using LKV Mili gama counter and Immunotech-IM1841 kit. Also, all subjects completed questionnaires for the acquisition of chronotype, quality of sleep and adaptation to shift work. Results: The mean age, work experience and BMI of the sample was 33.5+6.2years, 6.2+2.5 years and 24.1+2.8, respectively. 89.5 percent of subjects were married. The mean cortisol level at the beginning of night shift, the end of night shift and the beginning of morning shift was 6.95 Mg/dl,18.31Mg/dl and 19.1Mg/dl, respectively. 83.9 percent of subjects had morning chronotype and 16.1 percent had evening chronotype. The mean cortisol level at the beginning of morning shift and at the beginning and end of night shift for morning and evening subjects were (19.5, 16.5),(6.5,7.7)and(17.1,18.4)Mg/dl. 46 percent of subjects reported good sleep at the morning shift, while others reported moderate and weak sleep. Also, at the night shift, only 21.4 percent showed good sleep. Conclusion: Although there was a difference between the mean cortisol levels of morning and evening active subjects, but because of small sample size of evening active subjects, statistical analysis showed no significant difference between them. The mean cortisol level at the beginning of morning shift was related to the quality of sleep directly. There was a significant difference between the mean cortisol level at the end of night shift and morning shift sleep score (P=0.035). Also spearman analysis revealed a significant correlation between morning shift sleep score and value of beginning night shift cortisol reduction per end of night shift. There was a correlation between mean cortisol level at the end of night shift and adaptation score. Results showed that as the adaptation score increased, the differences between mean cortisol level at the beginning and the end of their night shift decreased.

Keywords: Shift work, Cortisol, Circadian rhythm, Sleep, Individual differences, Chronotype

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