PROJECT SUMMARY/ABSTRACT Children’s screen media use has increased dramatically in recent years. Greater screen use, especially during the evening hours, is routinely, though not unequivocally, associated with disruption of sleep including shorter sleep duration, difficulty falling asleep, and reduced overall sleep quality. Based in a wealth of research showing poor sleep health in childhood to robustly forecast a wide range of adverse outcomes (e.g., obesity, psychiatric disorders, poor cognitive and academic performance, emotion dysregulation, increased risk-taking behaviors, suicidality), current guidelines provided by virtually every major pediatric health organization/association recommend that children avoid use of electronic screens in the hour before bed. While perhaps intuitive, empirical evidence to support this guideline is largely cross-sectional. That is, causal effects of evening screen use on children’s sleep and circadian timing have not been sufficiently demonstrated nor have the mechanism(s) through which sleep disruption may occur been clarified. Experimental studies are therefore necessary to provide professionals and parents with evidence-based guidance and inform developmental research. The goal of the current study is to systematically test the impact of light exposure from screens as well as arousing media content on N=200 school-aged children’s (8 to 11 years) sleep regulation and circadian timing, in addition to next-day emotion regulation and executive functioning. Our novel ability to objectively assess and account for children’s daytime tablet use history will allow us to accurately isolate the effects of evening screen use on children’s sleep. Using a 4-group randomized, controlled design including assessment of typical sleep and media use followed by a 3-day experimental protocol, we plan to: 1) systematically test the effects of evening screen media use in the natural home environment on children’s sleep duration, sleep latency, and subjective sleep quality; 2) translate experimental research methods from the laboratory to the home environment to examine the impact of bright light emitted from screens and exposure to arousing media content on children’s sleep, circadian physiology, and physiological arousal; 3) examine the role of evening screen use on children’s next day executive functioning and emotion regulation. The proposed research addresses several critical gaps in scientific knowledge regarding the impact of evening screen media use on children’s sleep health, including elucidating the specific mechanism(s) through which potential negative impacts may occur. Results may directly inform national guidelines and policies regarding evening screen media use in all children.