This study investigates whether the psychophysiological benefits of breathing exercises are driven by a specific physiological rhythm (6 breaths per minute) or by the general psychological experience of performing a structured, mindful activity. Researchers aim to determine if "coherent breathing", which is hypothesized to synchronize heart and respiratory rhythms, offers unique physiological protection against stress compared to breathing at a natural pace or simple resting. The main questions it aims to answer are:
* Does slow, steady breathing at 6 breaths per minute lower physical stress markers (like heart rate variability and cortisol) better than faster, but structured breathing or just sitting still? * Is the calming effect caused by the specific breathing rhythm or simply by performing a structured, relaxing activity?
Researchers will compare three groups to see if the specific rhythm of "coherent breathing" offers unique benefits:
1. Group (Interventional): Coherent Breathing: Slow breathing at 6 breaths per minute. 2. Group (Sham Breathing): Regular breathing at 15 breaths per minute (matching a natural pace). 3. Spontaneous Breathing (Control Group): Natural, unguided breathing.
Participants will:
* Complete a one-day preparation phase to become familiar with the breathing technique. * Visit the research center for one experimental session. * Perform their assigned breathing method before and after a stress test. * Take the Maastricht Acute Stress Test (MAST), which involves putting a hand in cold water and doing mental math. * Provide saliva samples and have their heart rate variability, and mood measured multiple times.
Healthy adults
Aged 18 to 60 years
Willingness to participate in all study phases, including preparation and laboratory session.
Professionally active individuals or university students.
Severe chronic diseases, including metabolic disorders (e.g., diabetes) and mental disorders.
Cardiac arrhythmia, history of heart attacks, strokes, or heart surgery.
Regular use of medications such as anxiolytics or beta-blockers (excluding hormonal contraception)
Pregnancy.
Parallel
Coherent Breathing (6 bpm)
BehavioralA 10-minute session of guided nasal diaphragmatic, coherent breathing. Participants follow rhythmic audio bell cues: a low tone for a 5.5-second inhalation and a high tone for a 5.5-second exhalation. Instructions emphasize silent, gentle nasal breathing and abdominal (diaphragmatic) expansion without lifting the shoulders. Participants are asked to focus their attention on the sensations of air flow or abdominal movement throughout the session. The intervention is performed twice: before and after the stress protocol
Sham Breathing (15 bpm)
BehavioralA 10-minute session of guided nasal diaphragmatic breathing at a rate of 15 breaths per minute (matching a natural spontaneous pace). Participants follow identical rhythmic audio bell cues as the experimental group: a low tone for a 2.0-second inhalation and a high tone for a 2.0-second exhalation. All other instructions regarding nasal breathing, abdominal mechanics, and mindful attention are identical to the experimental group to ensure structural blinding. The intervention is performed twice: before and after the stress protocol
Change in Root Mean Square of Successive (RMSSD)
The Root Mean Square of Successive Differences (RMSSD) measured in milliseconds (ms). This is a time-domain HRV parameter used to assess parasympathetic nervous system activity. An increase indicates higher parasympathetic activity, while a decrease indicates a stress response.
Change in PNS Index
This outcome measure is reported as a single composite score (standardized Z-score) representing the aggregated modulation of vagal activity. The final value is automatically calculated by Kubios HRV Scientific software from the mathematical combination of three specific assessments: mean RR interval, RMSSD, and the non-linear SD1 parameter, normalized against a reference database. Only the final aggregated Z-score will be reported as the endpoints for these measures. A value of 0 represents the population average; positive values indicate enhanced parasympathetic tone and recovery capacity.
Change in Salivary Cortisol Concentration
Concentration Measured via enzyme-linked immunosorbent assay (ELISA) in nmol/L. This neuroendocrine marker assesses the activation of the hypothalamic-pituitary-adrenal (HPA) axis. It is used to quantify the objective impact of the breathing intervention on the peak hormonal stress response (typically reaching maximum concentration 15-30 minutes after the stressor).
Change in STAI-State score
A validated self-report questionnaire used to measure the current intensity of anxiety, tension, and nervousness. Scores range from 20 to 80, with higher scores indicating greater subjective psychological distress.
Change in SNS Index
This outcome measure is reported as a single composite score (standardized Z-score) representing the aggregated physiological arousal and sympathetic activity. The final value is automatically calculated by Kubios HRV Scientific software from the mathematical combination of three specific assessments: mean heart rate (HR), Baevsky's Stress Index, and the non-linear SD2 parameter, normalized against a reference database. Only the final aggregated Z-score will be reported as the endpoints for these measures. Positive values indicate sympathetic dominance and physiological arousal, providing a quantitative measure of the "cost" of the stressor and the effectiveness of the intervention in reducing sympathetic overdrive.
Change in Diastolic Blood Pressure (DBP)
Measured in millimeters of mercury (mmHg) using an automated sphygmomanometer. This metric serves as functional hemodynamic endpoint to verify the hypothesis that coherent breathing enhances baroreflex sensitivity and stabilizes blood pressure.
Change in Subjective Stress (VAMS)
A specific assessment of subjective stress intensity. Participants mark their current feeling on a 100 mm visual analogue scale (0-100), where 0 indicates "no stress" and 100 indicates "maximum stress" . Only this individual scale value is reported as the endpoint for this measure.
Mean Heart Rate (Mean HR) during Experimental Blocks
Continuous monitoring of mean heart rate (bpm) during active interventions and stress induction to evaluate real-time physiological reactivity and recovery dynamics.
Depression Anxiety Stress Scales (DASS-21)
A 21-item self-report instrument used to characterize the participants' baseline psychological distress over the past week. Subscales for Depression, Anxiety, and Stress (scores 0-21 each) will serve as covariates to analyze the interaction between baseline state and intervention results.
State-Trait Anxiety Inventory (STAI-Trait)
Measures the participant's stable tendency to respond with anxiety. Scores range from 20 to 80. This trait measure will be used to analyze how personality-level anxiety influences the physiological response to the stressor and breathing tasks.
Respiratory Rate (RESP)
Objective verification of respiratory frequency (breaths per minute) derived from RR interval data using the Kubios RESP algorithm. This measurement is used as a manipulation check to confirm participant adherence to the assigned breathing protocols (6 bpm for the experimental group and 15 bpm for the sham group) and to ensure the internal validity of the experimental blocks.
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