Analysis of salivary cortisol levels to determine the association between depression level and differences in circadian rhythms of shift-working nurses: Michie BABA, et al. School of Nursing, Faculty of Medicine, Fukuoka University-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