Background:

Vagus nerve stimulation has been shown to influence the hypothalamic–pituitary–adrenal (HPA) axis and autonomic nervous system activity. Non-invasive transcutaneous approaches may provide a feasible method to modulate physiological stress markers and related psychological symptoms. However, the relative effects of unilateral versus bilateral tVNS remain underexplored.

Methods:

Forty healthy adults were recruited from a primary care center and provided informed consent; 37 participants (8 males, 29 females, aged 21–64) completed the study. Hair samples (1-cm segments from the posterior vertex) were collected before and after four weeks of tVNS to assess cortisol and cortisone accumulation. Participants completed three validated questionnaires—Generalized Anxiety Disorder 7-item scale (GAD-7), Patient Health Questionnaire-9 (PHQ-9), and Pittsburgh Sleep Quality Index (PSQI)—at baseline, two weeks, and four weeks. Participants received either unilateral or bilateral stimulation using a Pulsetto device. Data were analyzed using Wilcoxon signed-rank tests, with subgroup analyses for unilateral and bilateral stimulation. Spearman correlations examined relationships between cortisol and cortisone changes.

Results:

Across the full sample, hair cortisol decreased significantly after four weeks of tVNS (V = 514, p = 0.013). Subgroup analyses indicated that this effect was significant for bilateral stimulation (p = 0.024) but not for unilateral stimulation (p = 0.159). Cortisone showed a trend-level reduction in the total sample (p = 0.059), with a near-significant effect in the bilateral subgroup (p = 0.058) and no change in the unilateral subgroup (p = 0.431). Cortisol and cortisone changes were moderately correlated (Spearman’s ρ = 0.50, p = 0.002).

Significant improvements were observed in all self-reported measures. Depression symptoms (PHQ-9) decreased in both unilateral and bilateral groups (p < 0.001), as did anxiety (GAD-7; unilateral p = 0.0004, bilateral p = 0.003) and sleep disturbances (PSQI; p < 0.001 for both subgroups).

Conclusion:

Four weeks of tVNS were associated with reductions in hair cortisol, particularly with bilateral stimulation, and with improvements in self-reported depression, anxiety, and sleep quality. Cortisol and cortisone changes were moderately correlated, suggesting partial convergence in HPA-axis modulation. These findings indicate that tVNS may influence both physiological and psychological stress markers, but further studies with larger samples are needed to confirm these effects and clarify the underlying mechanisms.

Funding:

This study was funded by UAB Pulsetto as part of an EU-supported project under grant number 05-001-01-05-07and conducted independently by Clinical Trial Center UAB INLITA. The company had no influence on the design, execution, or analysis of the study.