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Previous research associated smartphone use with worsened sleep among adolescents. However, the prior findings were mainly based on cross-sectional, self-reported data, and a between-person level of analysis. This study examined between- and within-person associations for adolescents' smartphone use and multiple sleep outcomes: sleep onset time, sleep onset latency, sleep duration, subjective sleep quality, and subjective daily sleepiness. The participants were 201 Czech adolescents (aged 13–17) who daily reported their sleep outcomes, daily stressors, and other media use for 14 consecutive days via a custom-made research app on their smartphones. The app also collected logs of the participants' smartphone use. We found that interindividual differences within the average volume of smartphone use before sleep were not associated with differences in sleep outcomes. At the within-person level, we …