Spaceflight often exposes crews to unusual sleep-wake and work schedules that lead to misalignment of the circadian pacemaker, including abrupt slam-shifts of work hours into the night, or entrainment to unusual day-lengths (e.g., 24.65 h Martian sol), resulting in poor sleep, impaired alertness, and increased risk of fatigue-related accidents. Untreated circadian misalignment results in sleep and wake occurring at the incorrect circadian phase which reduces sleep quality and quantity and impairs alertness, reaction time, and cognition. Even without circadian misalignment, sleep duration is usually poor (~6 h/night) during spaceflight and hypnotic medications and caffeine are commonly used to address insomnia and daytime sleepiness. In such a high-risk environment as the International Space Station (ISS), the risk of sleepiness-related performance decrements and accidents must be minimized. Light is a potential powerful countermeasure for both circadian misalignment and sleepiness associated with spaceflight. The effects of light are non-pharmacological and safe and can be obtained from ambient lighting. Recently, we and others have shown that short-wavelength (blue) light is most effective for phase-shifting the circadian pacemaker and enhancing alertness and performance. In addition to permitting vision, manipulation of short-wavelength blue light in particular has the potential to be a safe, non-pharmacological countermeasure to reduce the risk of circadian misalignment and performance deficits during spaceflight. These benefits can be achieved either by enhancing blue light to increase circadian resetting and alerting responses when required, for example when adapting to a slam-shift, or depleting blue light to minimize the effects of light when these responses are not required, for example prior to sleep. Using blue light per se, however, is undesirable as it impairs visual function, an important consideration in spaceflight. Manipulation of the blue light content of white light has similar benefits, however, and provides a way to optimize both the visual and non-visual responses simultaneously. A unique opportunity has arisen with the need to replace the current lighting aboard the International Space Station. NASA has proposed a new solid state lighting system with three pre-determined settings to address different operational needs: 1) white light for general vision; 2) blue-enriched white light to enhance high circadian adaptation and alertness; 3) blue-depleted white light to minimize alertness prior to sleep. We have developed a Dynamic Lighting Schedule (DLS) which determines when each of these three settings will be used to optimize lighting to maintain visual function and as a countermeasure to facilitate circadian adaptation, improve alertness and performance, and enhance sleep. The current proposal will study how the new lights would be deployed to address the problems associated with a simulated slam shift, in a high-fidelity analog of the ISS lighting environment, sleep patterns, and work schedule. In two 8-day randomized clinical trials, we tested the hypotheses that deployment of the DLS could increase the rate of circadian adaptation, improve sleep, and improve cognitive performance, subjective alertness, and objective EEG correlates of alertness. Experiment 1a examined 8-hour phase advance shifts and compared the effect of a continuous 6.5 h exposure to the DLS following a gradual versus immediate ('slam') shift in the sleep-wake cycle (n=18). Experiment 1b examined 8-hour phase delay shifts and compared the effect of a continuous 8 h exposure to the DLS to a standard lighting control following a slam shift in the sleep-wake cycle (n=12). Analysis of Experiment 1a showed that there was no statistical difference in phase shifting between gradual and slam advancing protocols, both achieving ~2.5 h of phase advance. Preliminary analyses of Experiment 1b suggest significantly greater phase delays induced by the DLS method compared to control. Analyses of the sleep and performance outcome measures are ongoing. These data will form the basis of operational lighting recommendations for use of the Solid State Lighting Assemblies (SSLA) aboard ISS when deployed in 2016-2017.