The term “Zeitgeber” (a German neologism translated as “time giver”) encompasses the concept of this natures clock capable of being perceived by the human body. The timing and organization of many internal phenomena are guided by this environmental “pacemaker,” resulting in the predictable periodicity of many biorhythms. Of the many potential atmospheric Zeitgeber, the most reliable indicator of environmental time is the regular diurnal alternation of light and darkness.
The endogenous nature of circadian rhythms has been long established. Studies in humans and other mammalian species involving complete isolation from environmental time cues (periodic variations of light, temperature, and humidity) demonstrated the presence of an internal “biological clock” localized in the brain. Under these circumstances all mammalians had a persistent rhythmicity in the rest-activity, body temperature, and other cycles. It is therefore the interaction between these markers of environmental time and the internal pacemaker(s) (capable of being reset) that determines the structure and rhythmicity of many physiologic biorhythms. Under most circumstances, however, this time-dependent interaction between the organism and its surroundings is bidirectional: changes in patterns of activity (ie, inversion of the sleep-wake pattern; sleep deprivation) or in the environment (ie, resulting from travel to another time zone or significant variations in daylight time) may result in the temporary disruption and/or resetting of endogenous biorhythms. buy flovent inhaler
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