This four-arm, single-blind randomized controlled trial examines the effects of four brief daily 5-minute practices over 28 days on psychological flexibility and physiological variables in Spanish-speaking adults. Participants will be randomly assigned to one of four groups: Box Breathing, Cyclic Sighing, Mindfulness focused on exhalation with the syllable "AH", or Conscious Cyclic Sighing. Outcomes include state and trait anxiety (STAI), positive and negative affect (PANAS), psychological flexibility (MPFI-24, Psy-Flex), and physiological variables (heart rate, respiratory rate, heart rate variability). Assessments are conducted at baseline, daily during the 28-day intervention, at post-intervention (day 31), and at 2-month follow-up.
Age between 18 and 65 years (inclusive)
Native Spanish speaker or fluent in Spanish (sufficient to understand audio instructions and complete psychometric questionnaires)
Stable internet access and an electronic device (smartphone, tablet, or computer)
Ability to provide informed consent and commitment to the 31-day protocol and 2-month follow-up
Active diagnosis of severe or chronic respiratory disease that prevents safe performance of breathing exercises (e.g., advanced COPD, severe uncontrolled asthma, cystic fibrosis)
Current diagnosis of severe psychiatric disorder (e.g., psychotic spectrum disorders, acute-phase bipolar disorder, severe major depression with suicidal risk)
Any acute medical condition contraindicated for voluntary alteration of breathing patterns
Parallel
Box Breathing
BehavioralIndividualized box breathing based on CO2 tolerance test. Participants perform a maximal exhale and record the duration (CO2 discard time) to determine their personal cycle interval: 3-4s (0-20s discard), 5-6s (25-45s discard), or 8-10s (50-75s+ discard). Equal-phase cycles of inhale/hold/exhale/hold repeated for 5 minutes daily. Focus on mechanical breath control only.
Cyclic Sighing
BehavioralDouble nasal inhalation (slow deep inhale + short top-up inhale to maximal lung capacity) followed by slow complete oral exhalation. Repeated continuously for 5 minutes daily. No attentional instructions.
Mindfulness AH Exhalation
BehavioralFocused-attention practice with eyes closed. Attention directed to forehead region between the eyes. Each exhalation accompanied by soft audible vocalization of syllable "AH". Distractions managed with internal label "thinking-thinking" followed by attention redirection. Practiced daily for 5 minutes.
Mindful Cyclic Sighing
BehavioralHybrid practice: double nasal inhalation (cyclic sighing mechanics) combined with slow exhalation vocalized as soft audible "AH". Mindfulness instructions include attention to breath sensations and use of "thinking-thinking" label upon distraction. Practiced daily for 5 minutes.
State Anxiety (STAI-E)
State anxiety measured with the State-Trait Anxiety Inventory - State subscale (STAI-E). 20 items rated on a 4-point Likert scale (0-3). Higher scores indicate greater anxiety. Spanish validated version (Guillen-Riquelme \& Buela-Casal, 2011).
Positive and Negative Affect (PANAS)
Positive and negative affect measured with the Positive and Negative Affect Schedule (PANAS). 20 items rated on a 5-point Likert scale. Two subscales: Positive Affect (PA) and Negative Affect (NA). Spanish validated version (Lopez-Gomez et al., 2015).
Psychological Flexibility (MPFI-24)
Multidimensional psychological flexibility and inflexibility measured with the Multidimensional Psychological Flexibility Inventory short form (MPFI-24). 24 items rated on a 6-point Likert scale assessing 12 Hexaflex processes. Spanish validated version (Navarrete et al., 2024).
Psychological Flexibility (Psy-Flex)
Unified psychological flexibility measured with the Psy-Flex scale. 6 items rated on a 5-point Likert scale. A modified daily version will be used during the intervention period. Spanish validated version (Navarrete et al., 2024).
Trait Anxiety (STAI-R)
Trait anxiety measured with the State-Trait Anxiety Inventory - Trait subscale (STAI-R). 20 items rated on a 4-point Likert scale (0-3). Higher scores indicate greater trait anxiety. Spanish validated version (Guillen-Riquelme \& Buela-Casal, 2011).
Resting Heart Rate (RHR)
Resting heart rate in beats per minute (bpm) obtained from participants' personal wearable devices (smartwatch/fitness tracker). Change calculated as the slope of a linear regression fitted to daily values over the 28-day period.
Heart Rate Variability (HRV)
Heart rate variability obtained from participants' personal wearable devices during deep sleep. Change calculated as the slope of a linear regression fitted to daily values over the 28-day period. Only available for participants with compatible devices.
Respiratory Rate (RR)
Respiratory rate in breaths per minute obtained from participants' personal wearable devices. Change calculated as the slope of a linear regression fitted to daily values over the 28-day period. Only available for participants with compatible devices.
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