PROJECT SUMMARY/ABSTRACT During the pandemic, children's technology and digital media (TDM) use, and in particular tablet use, has increased dramatically. Greater TDM use has been associated with less healthy sleep habits including shorter sleep duration, later sleep timing, difficulty falling asleep, and poorer sleep quality. This is concerning as shorter sleep duration, later sleep timing, and poorer sleep quality in childhood have been linked with the development of obesity and associated conditions, poorer executive functioning (EF) and academic performance, and increased risk for anxiety and depression in adolescence. One of the most likely mechanisms through which TDM use affects sleep is through the contribution of evening light exposure. Exposure to light at night sends an alerting signal to the circadian system, signaling that it is daytime, suppressing the release of melatonin, a hormone that signals to the body that it is time to sleep. The pervasive use of mobile devices (e.g., tablets) among children may pose an even greater impact on children's circadian system due to the closeness of the device to the eye. To mitigate risks associated with TDM use and sleep problems, parents need practical guidance about the potential impact of TDM use on their children's circadian rhythms and sleep, especially in the evening when children are most sensitive to the light emitted from screen devices. The goal of Project 3 is to systematically test the effect of TDM use before bed on the sleep, circadian phase, and EF of 4-year-olds using a 3-group randomized controlled trial in which children are assigned to receive one of 2 experimental conditions (Group A:1 hour of tablet use in the hour before bed; Group 8: 1 hour of tablet use 2 hours before bed) or a control condition (no evening TOM use). The proposed research addresses several gaps in scientific knowledge regarding the impact and timing of evening TDM use in the home environment. First, the impact of the timing of TDM use relative to bedtime in children is unknown. Second, the extent to which evening TDM use affects children's circadian phase, sleep, and executive functioning above and beyond the effects of household lighting is unknown. Our findings will address these gaps in the literature and have direct implications on the guidelines for evening TDM use among children. Our novel ability to objectively assess and control for children's daytime TOM use and assess treatment fidelity using FLASH-TV and apps for assessing tablet use allows us to conduct this experimental study in the home environment. Due to these innovations, the proposed study will be the first to: 1) translate experimental research methods from the laboratory to the home environment to examine the impact of evening light exposure on children's circadian physiology; 2) examine the impact of evening TDM use in the home environment on the circadian timing of preschoolers; 3) systematically test the effects of evening TDM use in the home environment on children's sleep onset and duration; and 4) explore the role of evening TDM use in the home environment on children's EF (inhibitory control and working memory).