Most research on adolescents’ nighttime mobile device use relies on self-reported data. Researchers have not exploited the ability of passive sensing tools to obtain objective measures of mobile device use and quantify app use at bedtime. The current pilot study employs the eMoodie app (installed on smartphones or tablets) to passively collect information on which other apps children are using, when, and for how long. The eMoodie app also prompts the nine young study participants (ages 10-12) to answer experience sampling questions about their mood and wellbeing. An Actigraph device assesses each participating child’s physical activity and sleep. Findings to date shed light on the feasibility and acceptability of using eMoodie to objectively assess young adolescents’ phone and app usage.