In practical terms, the functional timing unit for the human appears to be approximately one day: the 24-hour cycle of alternating light and darkness. Many biorhythms exhibit a periodicity that approximates 24 hours. The term circadian encompasses the chronologic variability of such biologic phenomena during a given time interval. Circadian (circa: about, dies: a day), pertaining to a period of about 24 hours, is applied especially to the rhythmic repetition of certain phenomena about the same time each day. The phasic secretion of cortisol and growth hormone and the cyclic variation of body temperature are classic examples of circadian biorhythms. buy flovent inhaler
At first hand, intuitive reasoning tends to correlate the day and night or more appropriately the sleep-wake cycle with the structure of the circadian variations of certain biologic rhythms. Despite the relative simplicity of this assumption, the time course of many physiologic phenomena nevertheless occurs with a predictable periodicity which is tightly linked to these external (environmental) cycles. However, organization of the human circadian timing system and its relationship with the environment appears to be more complex. Inherent to the circadian concept is the need for an accurate perception of a reliable indicator of environmental time. This precise dynamic interaction between external and internal rhythms implies the ability to recognize and quantitate time.
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