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.
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State anxiety score
Timeframe: From baseline (day 1) to post-intervention (day 5)
Depressive symptom score
Timeframe: From baseline (day 1) to post-intervention (day5)
WHO-5 well-being score
Timeframe: From baseline (day 1) to post-intervention (day 5)
Change in RMSSD
Timeframe: from baseline (day 1) to post-intervention (day 5)
Change in skin conductance level
Timeframe: From baseline (day 1) to post-intervention (day 5)
Change in startle response amplitude
Timeframe: From baseline (day 1) to post-intervention (day 5)
Change in frontal midline theta band power at Fz
Timeframe: from baseline (day 1) to post-intervention (day 5)
Change in frequency of intrusive-thought-related lexical occurrences
Timeframe: from baseline (day 1) to post-intervention (day 5)
Change in SDNN
Timeframe: From baseline (day 1) to post-intervention (day 5)
Change in high-frequency power
Timeframe: From baseline (day 1) to post-intervention (day 5)
Change in low-frequency power
Timeframe: From baseline (day 1) to post-intervention (day 5)
Change in LF/HF ratio
Timeframe: From baseline (day 1) to post-intervention (day 5)
Change in PNS index
Timeframe: From baseline (day 1) to post-intervention (day 5)
Change in SNS index
Timeframe: From baseline (day 1) to post-intervention (day 5