IL6 and depressive symptom measurement: This stability likely reflects the biological basis of these measurements: HRV captures autonomic dysregulation, which is a core neurobiological feature of depression (Kemp et al., 2010), while biomarkers such as inflammatory cytokines (CRP, IL-6) and cortisol reflect HPA axis dysfunction consistently documented in depression (Sharma & Verbeke, 2020; Łapińska et al., 2025).