Effect of #LIFEGOALS on Adolescents' Mental Health

An overall measure of well-being and health-related quality of life will be assessed with the KIDSCREEN-10 (Erhart et al. 2009), consisting of 10 items scored on a 5-point Likert scale. The scale explores the level of the adolescent's physical activity, energy and fitness, depressive moods and stressful feelings, opportunities to structure and enjoy social and leisure time and participation in social activities, quality of interaction with parent and feelings toward parents, nature of the relation with other adolescents, and cognitive capacity and satisfaction with school performance. Sum scores range between 10 and 50, and a higher score indicates better health-related quality of life.

Average time of physical activity (PA) level (light, moderate or vigorous PA) per weekday/weekend, in minutes.

Minutes of light, moderate and vigorous PA per day are objectively measured via Axivity trackers (accelerometers) worn at the wrist. Change in the average of light, moderate and vigorous PA from pre to post is examined for weekdays and weekend. Higher values represent more PA.

Average time seated per weekday/weekend, in minutes. Minutes of SB per day are objectively measured via Axivity trackers (accelerometers) worn at the wrist. Change in the average sedentary time from pre to post is examined for weekdays and weekend. Higher values represent more SB.

Average time asleep per night, in hours. Sleep duration per night is objectively measured via Axivity trackers (accelerometers) worn at the wrist. Change in the average of sleep duration from pre to post is examined for nights preceding a weekday and weekend. Higher values represent more sleep.

Sleep Quality: Average sleep quality over the past week Self-reported sleep quality will be measured in the online questionnaire via four items assessing subjective experience of sleep quality, sleep latency, sleep interruption, and daytime sleepiness. The items are based on items from 2017/18 survey of the Health Behavior in School-Aged Children (HBSC) study (Inchley, Currie, Cosma, \& Samdal, 2018) and from the Adolescent Sleep Wake Scale (ASWS; Essner et al., 2015), and are scored on a 5 point Likert scale (totally disagree - totally agree). A sum-score will be calculated (ranging from 4 to 20) with higher scores indicating better perceived sleep quality.

Frequency of taking breakfast, in events per week. This is assessed via 2 items from the HBSC 2017/18 study protocol (Inchley et al. 2018): one for weekdays and one for the weekend. A sum score (ranging from 0 to 7) will indicate the average days of breakfast intake per week for the last month.

The subscale Psychological Well-being of the KIDSCREEN-52 will assess positive emotions and satisfaction with life. This subscale consists of 6 items scored on a 5-point Likert scale. A higher sum score (ranging from 6 to 30) indicates more happiness and more positive and satisfying views of life.

The subscale Moods \& Emotions from the KIDSCREEN-52 will assess the extent to which the adolescent experiences depressive moods and emotions and stressful feelings. This subscale consists of 7 items scored on a 5-point Likert scale. A higher sum score (ranging from 7 to 35) indicates a more positive mood and feelings.

The subscale Self-Perception from the KIDSCREEN-52 will be used to assess the perception of oneself as well as one's appearance (body image). This subscale consists of 5 items scored on a 5-point Likert scale. A higher sum score (ranging from 5 to 25) indicates greater self-confidence and self-satisfaction, and a more positive body image.

The subscale Social support \& Peers from the KIDSCREEN-27 will be used to assess the quality of interaction with friends and peers, as well as the perceived support. This subscale consists of 4 items scored on a 5-point Likert scale. A higher sum score (ranging from 4 to 20) indicate feeling more accepted and supported.

Resilience will be measured by the Brief Resilience Scale (BRS) (Smith, Dalen, Wiggins, Tooley, Christopher, \& Bernard, 2008). The BRS consists of 6 items scored on a 5-point Likert scale, assessing the capacity to bounce back after hard times. A higher sum score (ranging from 6 to 30) indicates greater resilience. We added a timeframe and adapted the wording of the Dutch version of the BRS (Soer et al., 2018) to make the items adequate for young adolescents. This was discussed in team, and the adapted questionnaire was tested on comprehensibility with 4 adolescents. The Dutch version of the BRS has shown sufficient reliability (α = .78) and construct validity in healthy workers (Soer et al., 2018).

The Custom Short Form of the Dutch version of the PROMIS Pediatric Bank v2.0 Depressive Symptoms v2.0 (PROMIS-PedDepSx; Irwin et al., 2010) will be used to assess depressive symptoms. This scale consists of 4 items scaled on a 5-point Likert scale. A higher sum score (ranging from 4 to 20) indicates more symptoms of depression. The PROMIS-PedDepSx showed good psychometric properties (Kaat et al., 2019).

The participants' impression of change in physical activity, sedentary time, sleep time, sleep quality and breakfast will be assessed in the online questionnaires using a one-item question for each behavior: To what extent do you do this [behavior] more or less since the start of the study? Answer options range from 1 (much less) to 5 (much more).

The participants' impression of change in physical activity, sedentary time, sleep time, sleep quality and breakfast will be assessed in the online questionnaires using a one-item question for each behavior: To what extent do you do this [behavior] more or less since the start of the study? Answer options range from 1 (much less) to 5 (much more).

Experiential Engagement with the narrative component of the #LIFEGOALS app refers to the user's experience when watching the episodes of the narrative.

Self-report items are based on the 'Digital Behavior Change Intervention (DBCI) engagement scale' (Perski et al., 2019). In consultation with author O. Perski, the items were adapted and limited to 3 items assessing interest, attention and affect. Items are scaled on a 5-point Likert scale. A higher sum score (ranging from 3 to 15) indicates stronger experiential engagement with the narrative.

Experiential Engagement with the remaining of the #LIFEGOALS app refers to the user's experience when using the app, not considering the narrative.

Self-report items are based on the 'Digital Behavior Change Intervention (DBCI) engagement scale' (Perski et al., 2019). In consultation with author O. Perski, the items were adapted and limited to 4 items assessing interest, attention, affect and usability. Items are scaled on a 5-point Likert scale. A higher sum score (ranging from 4 to 20) indicates stronger experiential engagement with the rest of the app.

Behavioral Engagement with the #LIFEGOALS app refers to the actual usage of the app.

It is assessed by logging user's activity in the app. Out of the system log data, the duration of usage will be extracted. Behavioral engagement will be reported as the average minutes of usage per week.