Objective: Everyday stressors elicit adaptive changes in the hypothalamic-pituitary-adrenal (HPA) axis and the autonomic nervous system. Data on the relationship between these two systems under real-life conditions are sparse. We, therefore, sought to examine the association between HRV and salivary cortisol, which were recorded simultaneously in a stress-exposed, prospective, occupational cohort.
Methods: The study population comprised 88 nurses. We recorded heart rate (HR) and HRV during 301 working shifts. Participants provided salivary cortisol samples at the beginning of their work shift and every 2 hours thereafter. Samples were collected during three investigation periods spread over 9 months. Change scores for cortisol were calculated as deviations from the expected circadian baseline. Change scores from the grand diurnal mean in the time domain-based root mean square of successive differences served to index alterations in HRV. To account for the temporal delay between changes in HR/HRV and changes in salivary cortisol, the latter were compared with the changes in HR/HRV observed 15 minutes to 45 minutes before the cortisol sampling.
Results: During periods of high stress as indexed by high cortisol levels, we found significant associations between cortisol levels and HR (r = .48, p < .001) and HRV (r = -.28, p = .05). However, during low stress periods, these associations were attenuated and became nonsignificant.
Conclusions: These data suggest a relative independence in the regulation of the HPA axis and the autonomic nervous system in response to everyday stressors but synchrony of both systems in highly stressful situations.