Altered Coordination Between Sleep Timing and Cortisol Profiles in Night-Working Female Hospital Employees

Introduction: The circadian cortisol rhythm is closely aligned with the sleep-wake cycle. Cortisol peaks within 30–45 minutes after waking up in the morning, gradually decreases throughout the day, reaching lowest levels around midnight before rising again prior to waking. This study aimed to determine and compare cortisol profiles in women working day and night shifts. Additionally, diurnal cortisol profiles were analyzed between two groups with different start times for day shifts.

Methods: A total of 68 female shift workers (mean age: 37 ± 10 years) and 21 female non-shift workers (mean age: 45 ± 10 years) from a German hospital participated in this cross-sectional study. Shift workers were monitored across two consecutive day shifts and three consecutive night shifts, while non-shift workers were monitored across two consecutive day shifts. Each study day included six to eight saliva samples collected during wakefulness, along with sleep timing data obtained via polysomnography and sleep diaries. Generalized additive mixed models were applied to estimate the non-linear relationship between time since waking up and log-transformed cortisol levels. Shift- specific cortisol curves were fitted and compared. In addition, the following summary measures were calculated: cortisol awakening response (CAR), cortisol peak-to-bed slope, and total cortisol output.

Results: In shift workers, cortisol curves differed between night and day shifts. On night shifts, a flattened U-shaped cortisol profile was observed after the post-awakening peak, whereas a normal diurnal cortisol profile, with decreasing levels throughout the day, was observed on day shifts in the same individuals. The shape of the cortisol curves was indicated in the peak-to-bed slopes: close to zero on night shifts (-0.02, 95%-CI = -0.82, 0.78) and steeply negative on day shifts (-2.57, 95%-CI = -3.17, -1.98). For shift workers, significant differences in cortisol curves between night and day shifts were observed during the following time periods after waking: 40 - 55 minutes, 2 - 12 hours, and 15 - 22 hours. Comparisons of day-shift cortisol curves between shift workers and non-shift workers showed no significant differences in the shape of the curves. However, differences were detected in mean cortisol levels at waking up (-0.41 log(nmol/L), 95%-CI = -0.79, -0.02) and in CAR (0.37 log(nmol/L), 95%-CI = 0.10, 0.63).

Conclusion: This study demonstrates that night shifts disrupt the alignment between cortisol rhythms and the sleep-wake cycle in female hospital employees. Cortisol secretion during night shifts remained lower, despite the physiological demand for higher levels to support work. Conversely, cortisol levels were elevated after night shifts when the women needed to rest. These findings highlight the physiological challenges associated with night work and the potential health and performance implications for shift workers. Support: This study was funded by the German Social Accident Insurance (DGUV Grant No. FF-FP0321). The funder had no role in study design, data collection, data analysis and interpretation, or decision to publish.

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Artikel online veröffentlicht:
08. Oktober 2025