In 2020, teenagers in Switzerland reported the highest increase in time spent online in the past decade, especially for smartphone and social media use. This increase is partly due to the changes in social habits induced by Covid-19 and further reflects the possible positive and negative consequences of digital media use in young people. Up to date, most research focused on the effects of digital media use on youth well-being conceptualized as the presence or absence of ill-being rather than on a positive conceptualization, like the presence of positive emotions and happiness (“hedonic” well-being) and the realization of one's intrinsic potential (“eudaimonic” well-being). This study, called HappyB, aims to investigate how digital media use, particularly smartphone and social media use, is linked, over time, to youth well-being conceptualized in a positive way. To better understand the underlying dynamics, this project includes two longitudinal studies in which young participants (about 1’600 students of four high schools in canton Ticino) will be followed over time. Data will also be collected through the use of the Ethica app. The project, funded by the Swiss National Research Fund (SNF), will start in January 2022 and will run for 24 months. The data collected will be analyzed at Harvard T.H. Chan School of Public Health (Boston, USA).