Objective: The aim of this study was to clarify whether there are differences in the circadian rhythms of shift-working nurses by assessing depression, fatigue and salivary cortisol levels. Methods: Forty nurses working in a two-shift system at “Hospital A”, Fukuoka City, Japan, used a self-rated depression scale (SDS) to assess their depression levels. Fatigue levels were measured with the visual analogue scale for fatigue (VAS-F); saliva was collected before and during shifts for three days. Results were analyzed with analysis of variance (ANOVA). Results: Thirty-six valid records were obtained, and subjects were classified according to SDS scores into a normal group (NG), moderate group (MG) and severe group (SG). There were no significant differences in the day shift salivary cortisol values of the three groups. However, the night shift salivary cortisol value for the SG was 0.132 µg/dl at 16:00, before starting the shift, and decreased to 0.036 µg/dl at 20:00. It increased slightly up to 0.057 µg/dl by 24:00 and formed a peak between 5:00 and 7:00, with the levels being 0.322 µg/dl and 0.305 µg/dl respectively. Meanwhile, the NG cortisol value was 0.154 µg/dl before the shift, decreased to 0.034 µg/dl by 20:00, slightly increased up to 0.093 µg/dl by 5:00 and presented its peak value, 0.253 µg/dl, at 7:00 next morning. Conclusions: SG nurses presented significantly increased salivary cortisol levels early in the morning during night shifts, showing a phase deviation in the circadian rhythm. Because subjective fatigue levels did not differ with time, SG nurses should understand and deal with physical changes in the early morning. This approach may reduce medical accidents and malpractice in the early morning.(J Occup Health 2015; 57: 237–244)

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