(1) Study Objectives: By investigating the change of cortisol levels during shift cycles among professional firefighters in Korea, this study aims to evaluate the difference between individuals’ stress response and the recovery of their circadian rhythm after working night shifts. (2) Methods: A total of 325 shift firefighters, who were working in 3, 6, 9, or 21 day cycles, participated in the study. Their urinary and serum cortisol levels were measured during the day (09–18), during the night (18–09), and every 24 h (09–09) per shift cycle, and adjustments were made for confounding factors. (3) Results: Serum cortisol levels were significantly increased after working during the night or for 24 h compared with that of working throughout the day. However, whether working night or 24 h shifts, the serum cortisol levels were undoubtedly different based on the 3, 6, 9, or 21 day cycles. In all shift cycles, the urinary cortisol level decreased during the night or throughout the 24 h shifts compared with sleeping during this time, but this was considered to be significantly applicable only to those working in 21 day cycles. Additionally, in serial measurements, the recovery of urinary cortisol secretion after a night or 24 h shift was successful for individuals working in 9 day cycles, but the recovery was delayed for those working in 6 or 21 day cycles. (4) Conclusions: After analyzing the urine cortisol levels, the study indicates that only subjects working in 9 day cycles fully recovered their circadian rhythm while those working in 6 day or 21 day cycles did not completely recover. Therefore, it is important to recognize how stressful night shifts can be, and it is crucial to enhance firefighters’ current shift cycles in order to allow sufficient recovery of their circadian rhythm as well as the prevention of disrupting their circadian rhythm after working at night. Further research is necessary to take into account the amount of work load, the challenges of being sleep deprived, and the individual’s capacity to overcome sleepiness.

中文翻译：

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韩国消防员按轮班周期比较皮质醇水平。

(1) 研究目的：通过调查韩国专业消防员轮班周期中皮质醇水平的变化，本研究旨在评估夜班工作后个体的压力反应和昼夜节律恢复之间的差异。(2)方法：共有325名轮班消防员参与研究，他们的工作周期为3、6、9或21天。他们的尿液和血清皮质醇水平在白天（09-18）、夜间（18-09）以及每个轮班周期每24小时（09-09）进行测量，并对混杂因素进行调整。(3)结果：夜间工作或工作24小时后血清皮质醇水平较白天工作显着升高。然而，无论是夜班还是24小时轮班，血清皮质醇水平在3、6、9或21天的周期中无疑是不同的。在所有轮班周期中，与睡眠期间相比，夜间或整个 24 小时轮班期间尿皮质醇水平均下降，但这被认为仅适用于 21 天周期工作的人员。此外，在连续测量中，对于以 9 天为周期工作的个体来说，夜班或 24 小时轮班后尿皮质醇分泌的恢复是成功的，但对于以 6 或 21 天为周期工作的个体而言，恢复速度较慢。（4）结论：通过分析尿液皮质醇水平，研究表明，只有以9天为周期工作的受试者才能完全恢复其昼夜节律，而以6天或21天为周期工作的受试者则没有完全恢复。因此，认识到夜班压力有多大非常重要，并且增强消防员当前的轮班周期至关重要，以便充分恢复其昼夜节律，并防止夜间工作后昼夜节律被打乱。有必要进行进一步的研究，以考虑工作量、睡眠不足的挑战以及个人克服困倦的能力。