The adaptation of man and other animals to a planet with a 24 hour period of rotation has resulted in the evolution of a circadian timekeeping system that organizes the physiology and behavior of organisms. During the invasion of land by tetrapods, EarthUs gravitational field became a major factor in evolution. It is thus not surprising that exposure to altered gravitational or temporal environments often results in physiological and/or behavioral dysfunction. In spaceflight nonhuman primates can exhibit a depression of body temperature, free-running circadian body temperature rhythms, and substantial changes in sleep-wake behavior. Physiological and psychological changes resulting from desynchronized circadian rhythms may significantly affect the health and well being of astronauts and cosmonauts. This project continues our investigation of the biomedical effects of alterations in gravity and lighting on metabolism and circadian rhythms of body temperature, heart rate, activity, feeding, drinking, and behavior of the rhesus monkey. These data may suggest interventions that would ameliorate the adverse effects of spaceflight and human conditions such as jet-lag or shift work. Activity, body temperature and heart rate were recorded from four juvenile rhesus during spaceflight. The subjects exhibited a delay in the timing of the body temperature rhythm with respect to the activity and heart rate rhythms, i.e. the peak in body temperature occurred later during spaceflight. In addition, there was a decrease in mean heart rate compared to ground control studies. These data support the hypothesis that the circadian timekeeping system is sensitive to changes in the gravitational environment. In addition, because multiple rhythms within an animal responded differently to the altered environment, these data support the hypothesis that the primate circadian timekeeping system contains multiple oscillators. *KEY*Biological rhythms, Temperature regulation, Gravity
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