The research examining the relationship between digital use and adolescent mental well-being boomed exponentially in the last decade. Despite important empirical progress, existing literature presents gaps in several directions. First, to date, there is a lack of micro-macro perspectives that jointly test how individual socioeconomic status and broader inequality contexts moderate the link between digital use and well-being. Previous studies tend to isolate either individual or family mechanisms from country-level patterns; few studies integrate them to identify where risks concentrate and why. Second, the mechanisms in this relationship are often explored with a single concept or a few concepts at a time. Yet, a holistic approach to underlying mechanisms that are rooted in theoretical reasoning remains underused to explain why some user profiles fare better than others. Third, intervention designs that can apply to wider audiences at once need further attention and evidence to pronounce their effectiveness. Finally, interventions that centre on individuals' agency tend to be investigated in (young) adults, with mixed findings on digital behaviours and adolescent mental health. Although schools are key developmental settings, rigorous evaluations targeting adolescents' digital practices and well-being remain limited and fragmented. This thesis tackles each of these gaps in the literature and examines the relationship between digital use and adolescent mental well-being with four complementary studies that span cross-national, person-centred, and intervention approaches.

Study 1 uses data from the Health Behaviour in School-aged Children study to explore the interplay between Problematic Social Media Use (PSMU) and socioeconomic inequalities in relation to positive and negative mental well-being outcomes. Results show that PSMU is linked to higher psychological complaints and lower life satisfaction, with stronger relationships among adolescents from lower Socioeconomic Status (SES). These inequalities are consistent across countries, but are strongest in countries with medium-inequality levels. This study demonstrates persistent socioeconomic inequalities in the relationship between problematic digital use and mental well-being, with the partial role of larger socioeconomic contexts in this association.

Study 2 investigates the distinct types of digital user profiles and whether adolescents with each type would differ in their mental well-being. Furthermore, this study explores the role of digital maturity, self-determined and competent technology use, as an explanatory mechanism. Adopting a Generalised Latent Class Analysis approach on primary data collected from Irish adolescents, the results indicate three digital user profiles: Light, Reserved, and Outgoing. Both Reserved and Outgoing users reported poorer mental well-being than Light users, but lower digital maturity fully explains the disadvantage of Outgoing users. Overall, this study emphasises the nuanced variations in how adolescents engage with digital devices, and how their outcomes (digital maturity and mental well-being) might differ accordingly. underscoring the importance of self-determined and competent technology use.

Study 3 aims to examine whether school-based interventions could be an effective strategy to address the excessive and problematic digital use and negative mental well-being outcomes. This study conducts a pre-registered systematic review and meta-analysis, showing that school-based interventions can reduce screen time, depressive symptoms, anxiety, as well as life satisfaction and psychological difficulties. However, evidence for effects on problematic digital use is more limited. This study affirms the critical role of school settings that can be utilised to improve adolescents' digital use habits as well as alleviate their mental health problems.

Study 4 tests the effectiveness of a digital detox intervention using a controlled field experiment design with an experience sampling method to capture the momentary associations. Specifically, this study examines whether doing a week of digital detox (i.e., setting a daily time off from phones) could reduce adolescents' (problematic) phone use and improve their affective well-being in the short term, and whether individuals' adherence to the protocol and subjective experience of the digital detox intervention could moderate the effectiveness of the intervention. The results indicate no overall change after the intervention week, with the exception that adolescents in the intervention group improved in coping with their problematic use symptoms; perhaps being informed about potential harms of excessive phone use helped the intervention group to recontextualise their problematic phone use tendencies. Additionally, adolescents who did shorter digital detox sessions reduced their phone use after the digital detox, suggesting that shorter, more feasible detox sessions could be more successful in changing phone use habits.

Together, this thesis indicates that (i) SES inequalities shape the well-being risks of PSMU, (ii) digital maturity is a key mediator of the association between digital user profiles and mental well-being, and (iii) school interventions hold potential -under certain conditions- in mitigating problematic digital use and supporting adolescent well-being.