Circadian rhythm sleep disorders affect a large number of people across a wide age range. Timed bright light exposure is a promising treatment for such sleep disorders that are thought to involve the biological clock. Yet, light treatment as currently practiced has significant drawbacks: The time- consuming and tedious nature of most light treatment regimens make them difficult for many people to use on a consistent and continuing basis. Moreover, the nature of the endogenous clock's response to light dictates that maximum effects are obtained at times when people are typically asleep. We have shown that the circadian clock responds to extraocular light in the same manner as when light is presented to the eyes. This finding offers potentially exciting solutions to problems currently complicating the therapeutic use of bright light. By eliminating the need to receive light via the retina, light delivery systems can be made more easily portable, and thus, less intrusive on users' behavior. As importantly, by eliminating the need to receive light through the eyes, treatments conceivably may occur while patients are asleep, thus taking advantage of the most optimal times of light exposure. Yet, before treatments can be successfully developed using extraocular sites, a better understanding of the basic properties of extraocular light exposure is required. This project will examine one of the most important issues in this regard the intensity of light required to induce clock resetting. In a counter-balanced, laboratory-based study, we will examine the phase shifting capacity of two light intensity levels in 12 healthy young adults. The study addresses an issue crucial to the successful implementation of light treatments using extraocular exposure, since demonstration of efficacy with reduced intensity levels could substantially enhance development of portable and affordable devices. PROPOSED COMMERCIAL APPLICATION: If the proposed device is effective in treating SAD, a simple, inexpensive, convenient unit would be commercially available for millions of individuals who suffer from this disorder. Such a unit could also be useful in treatment of other seasonal psychiatric disorders, circadian rhythm disorders such as sleep syndromes, and in some instances non-seasonal depression.