Blood pressure will be assessed in a supine position after a rest period of five minutes by use of a validated automated oscillograph (Mindray VS-900, Mindray, Darmstadt, Germany) for use in children and adolescents (Wong et al., 2006) Cuff size will be selected based on the individual circumference of the upper right arm according to the guidelines (Flynn et al., 2017; NHBPE Program, 2004). Blood pressure is measured 3 times with 2 min apart, whereby the last two are averaged and the first is discarded.

To measure psychological acceptance, participants are asked to complete the Acceptance and Fusion Questionnaire for Youth (AFQ-Y). This instrument assesses the willingness to experience private events, values-oriented action, experiential avoidance, and cognitive and emotional fu-sion. The Difficulties in Emotion Regulation Scale (DERS-36) is used to measure difficulties in emotion regulation. Additional information on sleep behavior, perceived stress, and well-being is gathered using the Insomnia Severity Index (ISI), the Perceived Stress Scale-Youth (PSS-Y), and the WHO-5 Well-Being Index.

Body weight and composition in light clothing will be conducted using the gold-standard method, dual-energy x-ray absorptiometry (DEXA), utilizing a GE Lunar iDXA® device (GE Lunar Inc., Madison, WI; software version 13.10). Participants in the study are required to fast for 6 hours prior to the assessment. The analysis will yield both absolute and relative metrics for total body fat mass and fat-free mass. Throughout the assessment, subjects will lie in a supine position (Marra et al., 2019). The effective radiation dose from a single whole-body DEXA (<10 μSv) is similar to the normal background radiation received over one day at sea level.(Shepherd et al., 2017)

The handgrip strength is measured with an adjustable application to accommodate the span of each hand size (Leonardo Mechanograph®, Novotec medical, Pforzheim, Germany). The measurement is performed with the arm held vertically straight, providing a safe and reliable measurement for children (Gąsior et al., 2020). The subjects are instructed to compress the ad-justable application as forceful as possible and hold the contraction for 5s. After a familiarization attempt, the measurement is performed three times with a rest of 1 min in between.

This self-report questionnaire assesses psychological acceptance in children and adolescents. It measures key components such as willingness to experience uncomfortable thoughts and emotions (acceptance), values-based actions, experiential avoidance, and cognitive/emotional fusion.

Scoring: The AFQ-Y consists of 17 items. Scale range: Responses are rated on a Likert scale from 0 ("Not at all true") to 4 ("Very true").

Interpretation: Higher scores indicate greater psychological inflexibility, which is considered a worse outcome.

This 36-item questionnaire measures various aspects of emotion regulation difficulties, including emotional awareness, clarity, acceptance, impulse control, goal-directed behavior, and access to regulation strategies.

Scoring: Each item is rated from 1 ("Almost never") to 5 ("Almost always"). Scale range: Total scores range from 36 to 180. Interpretation: Higher scores indicate more severe difficulties in emotion regulation, reflecting a worse outcome.

The ISI assesses the nature, severity, and impact of insomnia. It includes questions on sleep onset, sleep maintenance, early morning waking, satisfaction with sleep, and impact on daily functioning.

Scoring: 7 items, each rated from 0 ("No problem") to 4 ("Very severe problem").

Scale range: Total scores range from 0 to 28. Interpretation: Higher scores indicate more severe insomnia symptoms, a worse outcome.

This scale measures the degree to which situations in one's life are appraised as stressful, specifically for children and adolescents.

Scoring: Items are typically rated on a 5-point Likert scale. Scale range: Depending on version, generally 0 to 40. Interpretation: Higher scores reflect greater perceived stress, a worse outcome.

This short questionnaire assesses subjective psychological well-being over the past two weeks. It focuses on positive mood, vitality, and general interest.

Scoring: 5 items, each scored from 0 ("At no time") to 5 ("All of the time"). Scale range: Raw scores range from 0 to 25, which are then multiplied by 4 to give a final score from 0 to 100.

Interpretation: Higher scores indicate better well-being, a better outcome. Scores below 50 may indicate reduced well-being or risk of depression.

Physical activity is objectively measured over a 7-day period using a wrist-worn accelerometer. This device collects data on:

Duration, frequency, and intensity of movement Data source: Integrated gyroscope and accelerometer sensors Interpretation: Data is analyzed in line with age-appropriate activity thresholds (e.g., moderate-to-vigorous physical activity levels)

Participants provide a structured recall of the types of physical activities they engaged in during different parts of the day over the same 7-day period. This qualitative input complements the accelerometer data and offers context about activity types and preferences.

This questionnaire assesses physical activity levels over the past 7 days, including walking, moderate activity, vigorous activity, and sitting time.

Scoring: Time spent in each activity type is used to estimate energy expenditure in MET-minutes per week.

Interpretation: Higher MET-minutes indicate higher physical activity, a better outcome.

To assess pubertal development, participants self-rate physical characteristics based on standardized illustrations and descriptions of secondary sexual characteristics (e.g., pubic hair, breast/genital development).

Interpretation: Tanner stages range from Stage 1 (pre-pubertal) to Stage 5 (fully mature), allowing categorization into early, mid, or late pubertal status.