Naturalistic daily illumination patterns are either absent or arbitrarily modified in many modern living, working and laboratory environments. Evidence points to deleterious effects of inadequate daily illumination on health, mood, sleep, and productivity. The day-night cycle is essential for adjustment of circadian physiological timekeeping, and traditional mimics in the laboratory -- involving sudden transitions between darkness and bright light -- pose unnecessarily shocking disruptions. We are developing an alternative system in which gradual naturalistic twilights are presented, closely representative of outdoor illumination patterns at any time of year and latitude on earth. The light source is attenuated continuously by means of a unique, comprehensive 24-hour computer algorithm and precision motor-control system that drives light-blocking vanes. (Disclosure Document #139208, U.S. Pat. Trdmrk. Off.) In Phase II of this project, we will refine the instrument for widely-varying scientific, industrial, and home-use applications. Demonstration research will test the utility of the system in five light-dependent biomedical applications: the treatment of seasonal depressive illness; sleep and activity patterns in early human infancy; circadian ingestive patterns in the laboratory rat; fetal development and maternal physiology in the sheep; and reproductive viability in the laboratory primate breeding colony.