사람을 비롯한 대부분의 동물들은 주변 환경의 영향을 받아 주기적인 생활을 영위하고 있다. 지구의 자전 및 공전으로 그리고 국소적으로 발생되는 주기적인 변화는 생명체의 행동으로 뚜렷하게 표출된다. 이러한 환경의 주기적인 변화는 오랜 기간의 진화과정을 통하여 생명체 내의 유전자로 각인되었으며, 이 유전자들은 발생 과정을 포함하는 성장 과정에서 리듬으로 발현된다. 환경의 변화는 결국 유전자로 정착하여 생체 시계로서 작용하며, 시상하부의 시신경교차상핵(supr

Most animals, including human beings, live in a cyclic pattern of lift that is influenced by the ambient changes of environment. The regular changes occurred by rotation of the Earth itself its revolving around the Sun, and the local environment, are reflected by the distinct behavior in the living organisms. These regular changes of environment have been imprinted into the genes within the living organisms through the evolutionary process over a long period of time. The genes are expressed by rhythms during the process of fetal development followed by growth. The environmental modifications ultimately are settled in genes, serving as a biological clock that is located putatively in the hypothalamus. Thus the biological clock governs a large number of rhythms and affects the time of birth and death lift expectancy, behavior, physiology, cell division, biochemical reaction, etc. The rhythms are readjusted to the changes of environmental cues. The biological clock has the great advantage of predicting and preparing the regular changes of environment.

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