Chronobiology is the study of biological rhythms. Circadian rhythms, characteristics of living systems that cycle with a relatively fixed and predictable period of about 24 hours, fall into this category of study. An organism's circadian clock is usually synchronized to the 24-hour Earth day, including the day's light-dark signals and temperature changes. Space flight experiments have shown that circadian rhythms persist in the absence of environmental time cues in space, indicating that clocks regulating circadian rhythms reside within the organisms. However, some rhythms can show abnormalities relative to the light-dark cycle in space. Of special concern to astronauts is how circadian patterns of various hormones and/or metabolic processes may be altered in reduced gravity.

Body temperature regulation, like sleep and activity patterns, shows a circadian rhythm. In humans, with the exception of fever episodes, the body temperature rarely varies more that 2° C from the baseline at any time. This normal functioning of the thermoregulatory system is essential for life. Some alterations in core temperatures have been observed during space flight and suggest that the thermoregulatory system is also modified by microgravity exposure.

Teaching Goals:

• Compare the causes and symptoms of acute versus chronic fatigue and sleep loss.
• Describe physiological parameters that follow a circadian cycle, define circadian desynchronization, and explain how this desynchronization impacts humans in space.
• Identify methods for increasing a crewmember's resistance to fatigue, sleep loss, or circadian desynchronization during space flight.
• Outline the physiological systems and responses to heat or cold exposure.
• Describe the thermal environments humans may be exposed to during space flight.
• Describe any thermoregulatory problems/issues encountered or anticipated in space missions.
• Describe how such factors as nutrition, dehydration, and stresses can impact thermoregulation in space flight.