The goal of this project is to determine the optimal lighting for use as a countermeasure to sleep and circadian disruption for astronauts. Sleep deficits that diminish alertness, cognitive ability, and psychomotor performance are a serious risk factor during space missions. Currently, over 45% of all medications taken in space are sleep aids (Putcha et al., 1999). Despite their use, more than half of the astronauts still get only between 3 and 6 hours of sleep per night during spaceflight (Barger et al., 2009). Light is the primary stimulus for human circadian regulation. On Earth, humans have a 24 h day/night cycle to maintain healthy circadian entrainment. In space, astronauts must contend with either rapidly changing or severely disrupted day/night cycles and must work in a dimly lit spacecraft interior with few windows. Bright white light has been implemented as a pre-launch countermeasure but has yet to be used during spaceflight. Providing sufficient light intensities to work areas in the International Space Station (ISS) as well as future vehicles and habitats raises several concerns, including heat production, energy consumption, and up-mass. Improving these factors requires a better understanding of how different light sources regulate the human circadian system. From these data it may be possible to optimize astronaut and ground crew light exposure both as a stimulus for vision as well as a countermeasure for sleep and circadian disruption during space missions. Currently, NASA uses white fluorescent light for interior illumination of the ISS at relatively low intensities, and for a pre-launch lighting countermeasure for circadian disruption at much higher intensities. In the previous funding period, our National Space Biomedical Research Institute (NSBRI) research was supported to determine if those fluorescent lamps have increased efficacy for melatonin suppression and circadian phase-shifting when they are enriched in the blue portion of the spectrum. Philips Lighting, an NSBRI industry partner, provided prototype blue-enriched lamps and exposure systems for that study. In addition, we began characterizing the neuroendocrine potency of ambient light on the surface of Earth for comparison to extraterrestrial light that astronauts will encounter on the surfaces of the Moon and Mars. To develop comprehensive lighting countermeasures for long duration space exploration, it is vital to determine the sensitivity of astronauts' circadian systems to both ambient and artificial lighting stimuli. In the current period of research, we shifted our efforts from testing fluorescent light sources to testing solid-state lights. Artificial illumination for future space vehicles and habitats will be provided predominantly by light emitting diodes (LEDs). The following list identifies the progress on our specific aims. 1. Assess the ocular safety of blue-enriched LED light at irradiances higher than those currently specified in space habitats. Working from national and international safety standards, hazard analyses were completed on blue-enriched lights to assure the ocular safety of our human volunteers. 2. Determine the potency of broad bandwidth LED light being developed for ISS as well as future space flight habitats. For this aim, we used 4' x 4' white LED lighting systems that emit white light with Correlated Color Temperatures (CCTs) of 4,000 K or 6,500 K. Two separate experiments with healthy subjects (N=8, each) and over 160 nighttime melatonin suppression studies have been completed. There was a distinct fluence-response relationship between light irradiance and melatonin suppression for both types of lamplight. 3. Evaluate selected polychromatic LED stimuli for supporting visual performance, color discernment, and neuroendocrine potency inside replicas of ISS Crew Sleeping Quarters (CQs). Replica CQs were illuminated by the ISS prototype solid state lamps (SSLM-R). Four separate nighttime melatonin suppression studies were completed within replicas of the CQs illuminated by white SSLM-R light with a CCT of 6,500 K, 4,800 K, or 2,700 K. These studies quantified the efficacy of the different light sources for neuroendocrine regulation. In addition, two studies were done on visual performance and color discernment within replicas of the CQs illuminated by different polychromatic light stimuli emitted by SSLM-Rs. Lastly, a multi-day pilot study, co-supported by NASA, assessed visual sensitivity, pre-sleep melatonin secretion, subjective alertness, objective alertness, neurobehavioral responses, and sleep parameters relative to different SSLA lighting stimuli in astronaut-aged males. 4. Assess the potency of ambient light on the Earth's surface for melatonin regulation. Astronaut-aged male and female subjects (N=8, 43.3 +/- 1.4 years) completed an Earth daylight study that used Ganzfeld exposures. The data show that simulated Earth daylight evokes a fluence-response curve for acute, nocturnal melatonin suppression. Together, the research from the two prior funding cycles along with the studies performed in the recent funding cycle provide significant progress towards the development of lighting countermeasures for sleep and circadian disruption in astronauts and ground crew members. Programmatically, it has provided information that was used in the revision of Constellation Program Human-Systems Integration Requirements (December, 2009). Further, the results will also impact the NASA Human Integration Design Handbook and the Space Flight Human Systems Standard, NASA-STD-3001, which provide guidance for crew health, habitability, environment, and human factors in human spaceflight. Our progress addresses Critical Risk areas 9 (EVA--extravehicular activity--7) and 22 (Sleep 5, 9, and 10) in the NASA Human Research Program Integrated Risk Plan (2009). These areas concern countermeasures that will optimally mitigate performance problems associated with sleep loss and circadian disturbances and the mismatch between crew physical capabilities and task demands.