CIRCADIAN RHYTHM STRESS

also called Synchronization Stress, temporary maladjustment caused by rapid change in the cyclical 24-hour period of human biological activity. Within the circadian (24-hour) cycle a person usually sleeps approximately 8 hours and is awake 16. During the wakeful hours, mental and physical functions are most active and tissue-cell growth increases. During sleep, voluntary muscle activities nearly disappear and there is a decrease in metabolic rate, respiration, heart rate, and blood pressure. The activity of the digestive system increases during the resting period, but that of the urinary system decreases. Hormones secreted by the body, such as the stimulant epinephrine (adrenaline), are released in maximal amounts about two hours before awakening so that the body is prepared for activity. The controlling mechanism for these cyclic activities, thought to be a region of the brain known as the hypothalamus, seems to be the master centre for integrating rhythmic information and establishing patterns. Body temperature patterns and the effectiveness of some drugs vary at different hours of the circadian cycle. The timing of administration of hormonal drugs so as to be in accord with their natural circadian production pattern seems to place less stress on the body and produce more effective medical results. The natural time signal for the circadian pattern is the change from darkness to light. Where daylight patterns are not consistent, as in outer space, regimented cycles are established to simulate the 24-hour day. If one tries to break the circadian rhythm by ignoring sleep for a number of days, psychological disorders begin to arise. The human body can learn to function in cycles ranging between 18 and 28 hours, but any variance greater or less than this usually causes the body to revert to a 24-hour cycle. Even in totally lighted areas such as the subpolar twilight zone, the body has regular cycles of sleep and wakefulness once the initial adjustment has been made. Any drastic shift in the circadian cycle requires a certain period for readjustment. Each individual reacts to these changes differently. With the advent of jet air travel, time changes present a much greater problem than ever before. Jet travel between Tokyo and New York City creates a time difference of 10 hours; it usually takes several days for the body to readjust to the new day-night pattern. Too-frequent shifts in circadian patterns, such as several transoceanic flights a month, can lead to mental and physical fatigue. Preflight or postflight adaptation can be achieved by gradually changing one's sleeping patterns to simulate those that will be necessary in the new environment. Astronauts first encounter rapid changes in the day-night cycle while in Earth's orbit. Beyond this, the void becomes a constant blackness. Ability to adapt to differing day and night cycles will undoubtedly be an important factor in future space travel.