Adolescents' daily affect predicts their mobile phone use (not the other way around).

Adolescence is a sensitive period of development marked by heightened affective reactivity and shifting sleep patterns. Yet how adolescents' daily emotional states and sleep intersect with their mobile phone use remains poorly understood, partly due to debates about directionality and the reliance on often cross-sectional and self-reported global measures of use. The present study advances the field by using objective, naturalistic data to differentiate active phone use (time with keyboard input) from passive use (remaining foreground app time without keyboard input), testing within-person associations between mobile phone use and adolescents' morning affect and sleep. We applied multilevel models to 3192 daily observations of 40 Android-using adolescents (ages 15-20; 65% male) collected over a 90-day period using the Effortless Assessment Research System. Results indicated that when adolescents reported higher-than-usual morning negative affect, they engaged in greater passive use the same day; no associations were found with positive morning affect. Similarly, adolescents' daily fluctuations in active and passive mobile phone use did not predict sleep that night or next-morning affect. These findings suggest that adolescents' morning negative affect may shape how they engage with their mobile phones throughout the day.