LasaiON is an Evidence-informed, Multimodal Self-regulation Program Designed to Reduce Emotional Distress and Strengthen Stress-regulation Skills by Combining Transcutaneous Auricular Vagus Nerve Stimulation (taVNS) With Breathing Supported by Heart-rate Variability Biofeedback (HRV-B; emWave).

The goal of this clinical trial is to evaluate whether the LasaiON program, which combines transcutaneous auricular vagus nerve stimulation (taVNS) with breathing supported by heart rate variability biofeedback (HRV-B; emWave), reduces emotional distress and improves stress-regulation processes in adults. The study will also assess the safety, tolerability, and feasibility of the intervention.

The main questions it aims to answer are:

1. Does the LasaiON program reduce emotional distress in adults? 2. What physiological, psychological, and verbal-cognitive changes are observed following the intervention? 3. What discomforts, adverse effects, or tolerability issues do participants experience during the program?

Researchers will use a prospective 2 × 2 factorial randomized controlled design to examine the effects of stimulation condition (active taVNS vs. sham taVNS) and training condition (active HRV-biofeedback vs. control training), as well as their potential interaction. Participants will be randomly allocated in equal proportions (1:1:1:1) to one of four groups: sham taVNS + control training, sham taVNS + HRV-biofeedback, active taVNS + control training, or active taVNS + HRV-biofeedback.

Participants will:

* undergo baseline assessments on Day 1 before the intervention; * complete 5 consecutive daily 60-minute sessions according to group allocation; * undergo post-intervention assessments on Day 5 after the final session; * complete psychometric, physiological, and verbal-cognitive assessments, including STAI, HAM-D, WHO-5, EEG, skin conductance, startle response, heart rate variability measures derived using Kubios HRV Premium, and daily five-word reports with valence ratings analyzed using IRaMuTeQ; * have adherence and tolerability recorded at each session using a checklist.