Psychological and Neurobiological Impact of a Retreat Based on Mindfulness and Compassion for Stress Reduction.

Blood samples: 20 ml collected in the standard tubes used in clinical haematology that contain EDTA as an anticoagulant. From this biological material, DNA and RNA will be extracted according to the standard molecular biology procedures, in order to subsequently measure DNA methylation (by EPIC arrays) and gene expression (RNA-seq), respectively.

Depression Anxiety Stress Scales (DASS-21), (Lovibond and Lovibond, 1995). The DASS-21 is a 21-item scale containing 3 subscales (Anxiety, Depression and Stress), with 7 items for each subscale. Participants are asked to rate each item using 4-point Likert scale (0=Not applicable to me; 3=Very applicable to me). Higher scores correspond to higher levels of severity of anxiety, stress, and depression.

Mathematical Stress Test: a standardized laboratory stress induction procedure (Kirshbaum et al., 1992). Individuals are asked to repeatedly subtract a given 2-digit number from a 4-digit number. Time is limited to 3 minutes and participants are told that this is a mental speed task and their performance will be evaluated. Stress levels will be evaluated with an affective adjective checklist (PANAS scales).

EEG resting state (alternating open and closed eyes). EEG resting state will be recorded with four 64-channel EEG devices with laptop and eego EEG recording software. The sampling rate will be 1000 Hz and the reference to be used will be 7Z of the equidistant layout.

The interventions will significantly change the EEG spectral profile of the resting state brain activity. We expect changes in the balance between Alpha (8-14 Hz) and Theta (4-8 Hz) frequency bands.

Cortisol samples 1.5 - 2.0 ml will be collected in Salivette® cortisol saliva sample collection tubes (Sarstedt, Germany) that contain sterile cotton plugs for sample absorption. From this biological material, levels of the principal stress corticosteroid, the hormone cortisol, will be assessed in order to subsequently measure the Diurnal Cortisol Slope and cortisol levels at awakening and bedtime.

Using saliva collection tubes, levels of the principal stress corticosteroid, the hormone cortisol, will be assessed in order to subsequently measure the Diurnal Cortisol Slope and cortisol levels at awakening and bedtime at three different moments of the retreat.

A 10-min guided test to retrieve one personally painful memory. Affect changes will be asssed before and after the task by using a checklist of affective adjectives (PANAS).

State Mindfulness Scale (SMS) - (Tanay y Bernstein, 2013). The SMS has 21 items with a response scale from 1 (not at all) to 5 (very well) to indicate how well the statements describe their current mindfulness state. Higher scores represent higher states of current mindfulness.

Meditation-Related Adverse Effects Scale - Mindfulness-Based Program (MRAESMBP) -Britton et al. (2018). This is an 11-item scale measuring the frequency of challenging or distressful experiences during meditation practice.

State Self-compassion Scale (SSCS), (Neff et al., 2021). It is a 6-item scale measuring compassion towards oneself using a 5-'point scale (1=Not at all true for me; 5= Very true for me). HIgher scores indicate higher levels of self-compassion.

We will use the Fear of Expressing Compassion towards yourself subscale from the Fear of Compassion Scale (Gilbert et al. (2011). This is a 15-item scale to assess individuals' difficulties to feel or express self-compassion. We by using a 5-point Likert scale (0=Don't agree at all; 4=Completely agree). Higher scores indicate higher levels of fear of compassion towards oneself.

A scale measuring current general positive and negative mood (PANAS). It includes 20 adjectives (10 positive, 10 negative) to assess current mood in a scale from 1 (Not all) to 5 (Extremely). Higher scores represent higher positive or negative mood states. We will use the sum of positive items and negative items to provide two separate overall scores.

Satisfaction With Life Scale (SWLS), (Diener et al., 1985). This is a 5-item scale assessing satisfaction with life. Higher scores indicate higher levels of satisfaction with life.

Patient Health Questionnaire (PHQ-15), (Kroenke et al., 2002). The PHQ-15 comprises 15 somatic symptoms from the PHQ, each symptom scored from 0 ("not bothered at all") to 2 ("bothered a lot"). A sum score will be used to analyze somatic symptoms.

State Difficulties in Emotion Regulation (S-DERS-21), (Lavender et al., 2017). This is a 21-item scale in which participants rate their difficulties to regulate their mood in a 5-point Likert scale (1=Not alll; 5=Completely). Higher scores indicate more difficulties in emotion regulation.

State-Trait Anxiety Inventory (STAI-20), (Spielberger et al., 1983)

DSM-5 sleep difficulties (APA, 2013). This is the single item included in the Self-rated level 1 Cross-Cutting Symptom Measure (DSM-5, APA 2013). It measures quality of sleep using a 5-point scale (0=Very good; 4= Very bad).

Client Satisfaction Questionnaire (CSQ-8)- Atkinson and Zwicq (1982). It measures satisfaction with the course of psychological interventions. Participants must rate their satisfaction on 8 items in 4-point scale. Higher scores indicate higher satisfaction with the program.

Experience Sampling Methods on measures of: mindfulness, mind wandering, compassion to others, self-compassion, wellbeing, richness of life, and utility of the contents of the program that day. These are 'ad hoc' single items, taken from well-validated measures, to have a quick daily assessment of psychological functioning. Each item is assessed in a 5-point Likert scale (0=Never; 4= Always).

Hedonic and Arousal Affect Scale (HAAS), (Roca et al., 2021). This is a 12-item scale measuring affective states by including adjectives low or high in arousal and positive or negative in valence. Participants are asked to rate their affective experiences in a 5-point Likert scale (0=Not at all; 4= Absolutely).

Measurement of cardiac activity during EEG sessions. EKG will be used in the preprocessing procedure to insulate electrophysiological artifacts due to heart activity. In addition, the heart signals will be assessed to obtain heart variability and heart rate indicators.

Measurement of respiratory activity during EEG sessions. Respiratory signals will be employed in an specific analysis where the EEG data will be split in two blocks (inhalation and exhalation).