PROJECT SUMMARY Late sleep timing and evening sleep problems in early childhood increase the risk for poor behavioral and health outcomes. Light is the primary zeitgeber of the circadian system, and even small amounts of evening light can suppress production of the sleep-promoting hormone melatonin and delay circadian timing. The spectrum of light – not just the intensity – may be a particularly relevant factor for determining sleep and circadian timing in early childhood for several reasons: (1) children are more sensitive to evening light exposure than adults due to differences in ophthalmological features, including larger pupils and clearer lenses that increase light transmittance; (2) young children show robust melatonin suppression (~90%) following evening bright light exposure; and (3) media use in preschoolers has increased dramatically in the last decade and is associated with late sleep timing and bedtime resistance. Central to this research are also data from older children, adolescents, and adults indicating the circadian system shows high sensitivity to short- wavelength blue light. To date, however, the role of light spectrum in young children’s sensitivity to evening light exposure and its differential impact on circadian physiology is unknown. This is the primary purpose of this F32 research project. In a within-subjects crossover design, participants complete a 10-day protocol twice. Healthy, good sleeping children aged 3.0-4.9 years maintain a stable sleep schedule for 7 days, followed by a 3-day in-home circadian assessment performed in dim-light conditions. Children experience two counterbalanced experimental conditions: a high CCT (5000K; blue-enriched) or low CCT (2700K; red-enriched) light exposure of 100 lux for 1-h before their scheduled bedtime. Following a 1-2 week “washout period”, the protocol is repeated with the remaining condition. We will examine the impact of light spectrum on melatonin suppression (Aim 1) and circadian phase shift (Aim 2). Dim light melatonin onset is assessed through saliva using our established protocol to determine % melatonin suppression and circadian phase delay. The results of this proposal will identify modifiable components of children’s evening light exposure (e.g. light spectrum and timing) to help parents set limits and make healthy choices about their children’s lighting environment. Results are likely to have high impact, as they will provide behavioral change recommendations that can minimize the negative impacts of evening light exposure on children’s sleep duration and quality. In addition to these research aims, this F32 Award will incorporate training in the measurement and analysis of physiological measures, advanced statistical approaches, grant and manuscript writing, dissemination of findings to the public, and lab management. The research and training proposed will be an important step in facilitating Dr. Hartstein’s transition into an independent scientist with a research...