Participants will receive educational but not SoC and MI-based materials about excessive digital use, IGD, and SMA. Such psycho-educational interventions are widely used in behavioral/mental health promotion and have been demonstrated to be effective in increasing knowledge and awareness of behavioral and mental problems. Based on a comprehensive literature review and our previous psycho-educational programs for behavioral/mental health promotion, the research team will design the psycho-educational materials. Themes include definitions, factors, and health consequences of excessive digital use, IGD, and SMA, information on healthy lifestyles, self-regulation skills, and help-seeking resources.

The intervention includes five modules based on the five stages of Stages of change. Chatbots will build up a rapport with the participants, monitor their stage with a few simple questions based on previous Stages of Change studies, and provide stage-tailored packages of response, advice, and feedback (one of the five modules) accordingly to enhance their motivation and self-efficacy each week. For those who click 'yes' to a question assessing the first stage of change (PC), Chatbot will make a record and automatically ask questions assessing the next stage; for those who click 'no', Chatbot will automatically send them the module corresponding to their SoC via WhatsApp. Each module will take 10-15 minutes. In total, it will take 1 hour to complete all the modules.

The intervention includes five modules based on the five stages of Stages of change. Chatbots will build up a rapport with the participants, monitor their stage with a few simple questions based on previous Stages of Change studies, and provide stage-tailored packages of response, advice, and feedback (one of the five modules) accordingly to enhance their motivation and self-efficacy each week. For those who click 'yes' to a question assessing the first stage of change (PC), Chatbot will make a record and automatically ask questions assessing the next stage; for those who click 'no', Chatbot will automatically send them the module corresponding to their SoC via WhatsApp. Each module will take 10-15 minutes. In total, it will take 1 hour to complete all the modules.

IGD symptoms are assessed using the Nine-Item Internet Gaming Disorder Scale-Short Form (IGDS9-SF) consists of nine questions that assess IGD symptoms, including preoccupation, tolerance, withdrawal, unsuccessful attempts to limit gaming, deception or lies about gaming, loss of interest in other activities, use despite knowledge of harm, use for escape or relief of negative mood, and harm. The response options for each item include "yes=1" and "no=0". The overall score of the scale ranges from 0 to 9, with a higher score implying a higher level of IGD symptoms. Participants with score ≥ 5 will be classified as having probable IGD.

SMA is defined as excessive use of social media with the addictive symptoms like mood modification, salience, tolerance, withdrawal symptoms, conflict, and relapse. The symptoms of SMA will be measured by the 6-item Bergen Social Media Addiction Scale (BSMAS) based on the six core components of addictive behaviors, including cognitive salience, tolerance, mood modification, difficulty in regulating use, withdrawal, and interference with role performance. The items are rated using a Likert type scale ranging between 1 (very rarely) and 5 (very often). A higher sum score in the BSMAS indicates a greater likelihood of being at risk of developing social media addiction. A cut-off score over 19 indicates probable SMA.

Composite outcome of IGD score and SMA score: we define a simple composite as the sum of the two outcomes. Because each outcome measure uses a different scale and has different statistical properties, we will standardize the two variables (i.e., scores of IGD and SMA) first, and then combine the two standardized scores of the outcomes into a composite.

Sedentary behavior is defined as any waking behavior such as sitting or leaning with an energy expenditure of 1.5 metabolic equivalent task (MET) or less. Sedentary lifestyles will be measured by the 10-item sedentary lifestyles questionnaire (SLQ) which has been used in Chinese school children. Subjects will report the average daily time spent (hours) during the weekdays and weekends in the following activities: Using smartphone or tablet PC for learning/doing paper homework or reading or writing/ sedentary leisure time without time for screen leisure. Average number of daily hours will be calculated as (total sedentary weekday hours × 5 + total sedentary weekend hours × 2) ÷ 7.

Autonomy involves feeling a sense of personal agency and volition such that one's behavior is perceived to emanate from an internal locus of causality. To assess autonomy to reduce dependence on Internet gaming/social media, the subscale of the Psychological Need Satisfaction in Exercise Questionnaire (PNS-A) will be adapted and used. The Chinese version was demonstrated to have good psychometric properties. The scale includes six items measuring psychological need fulfillment of autonomy. Items are rated on a 6-point Likert scale, ranging from 1 (false) to 6 (true). Total scores range from 6 to 36, with higher scores being indicative of higher degrees of autonomy.