There is growing concern that smartphones and social media (S/SM) disrupt adolescent sleep, wellbeing and cognition, yet causal evidence from robust interventions is limited. To assess the acceptability of a total 21-day S/SM detox we conducted a preregistered (https://osf.io/79zd2) pilot randomised control trial with eighty-two adolescents aged 13-18 years from UK schools. Participants were randomly assigned to a total S/SM detox (N=26), total S/SM detox with access to a basic “brick” phone (N=26) or a business-as-usual control (N=30). Pre-and post-intervention measures of sleep, wellbeing, and cognition were collected using surveys, wearables, and cognitive tasks. Retention was high, with 13% attrition across groups. At 21-days, one or both detox groups showed tentative improvements in sleep duration, daytime sleepiness, depressive symptoms, perceived stress, and working memory relative to the controls, however these changes were not maintained at a two-month follow-up. Open-ended post-detox reflections reflected a range of both positive and negative experiences, with reported challenges including boredom, functional reliance on smartphones, and fear of missing out. Nevertheless, 83% endorsed the need for government action. Digital detoxes appear feasible for generating causal evidence; however, they can also pose functional challenges to young people. Fully powered trials should address these challenges, potentially via more targeted intervention approaches (e.g. switching off the internet on smartphones, particularly at bed-time).