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Circadian Rhythm Living organisms on this planet have adapted to the daily rotation of the earth on its axis By means of endogenous circadian clocks that can be synchronized to the daily and seasonal changes in external time cues most notably light and temperature Most people anticipate environmental transitions perform activities at biologically advantageous times during the day and undergo characteristic seasonal responses The effects of transmeridian flight and shift work are stark reminders that although modern technologies can create cities that never sleep we cannot escape the recalcitrance of endogenous clocks that regulate much of our physiology and behavior Recent progress in understanding the molecular mechanisms underlying circadian rhythms has been remarkable In its most basic form circadian clocks are comprised of a set of proteins that generate a self-sustaining feedback loop with a free-running period of about 24 hours One or more of the clock components is acutely sensitive to light thus it can be synchronized to local time Edery 2000 Levels of light and the Circadian Rhythm The sleep-wake cycle or circadian rhythm in people is largely governed by exposure to light Bright light enters the eye and follows the optic nerve to where the nerves for both eyes cross and then stimulates a small bit of tissue that begins a cascade of chemical and nervous events stimulating wakefulness In particular a substance called melatonin is prevented from being produced by the stimulation of light When people are exposed to low light levels or darkness as in a Com Center or radar ARTCC room the brain interprets this as sleep time and begins to produce melatonin and other substances that tell the body to begin to shut down This change in the body clock has both physiological and psychological effects The digestive system slows secretion of digestive juices coordination and reaction time slow body temperature drops and general alertness and level of arousal decrease Fighting this physiological switch
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