Effects of Shift Work Type on Sleep Quality and Cortisol Regulation Among Nurses: A Prospective Observational Study

Rotating shift work is a common occupational stressor among healthcare professionals. This study aimed to examine the impact of shift work on sleep quality and cortisol levels among nurses. A prospective observational study was conducted in Croatia in 2025, involving 140 nurses. Sleep quality was assessed using the Fitbit Charge 3 smartwatch over eight consecutive days, and blood cortisol was measured at two time points. Most nurses (98.6%) had a neutral chronotype. Nurses who work day shifts (DSN) achieved significantly higher sleep scores compared with rotating-shift nurses (RSN) (77, IQR 75–80 vs. 73, IQR 68–76; Mann–Whitney U test, p < 0.001). RSN also had significantly higher morning cortisol levels at the first measurement (median 431.6, IQR 351.3–496.2 vs. 355.15, IQR 254.9–434.2) nmol/L; p < 0.001). In the RSN group, morning cortisol levels at the first measurement were positively correlated with age and years of service, and negatively correlated with circadian rhythm. Smartwatch-based assessments demonstrated that rotating shift work is associated with poorer sleep quality and transient disruption of cortisol regulation. Notably, cortisol levels in rotating-shift nurses returned to levels comparable to those of DSN after two nights of rest, indicating substantial reversibility of the hormonal response. These findings highlight the short-term physiological impact of shift work and underscore the importance of optimizing work schedules and implementing strategies to reduce long-term health risks.