The term biological clock is appied to the means by which living things adjust their activity patterns, without any obvious cue ,the time of day, or the year. The biological clocks seem to be beautifully adapted to day need of living things. They are affected but little, if at all, by drugs, chemicals. Or wide temperature differences - - factors which may alter substantially the rate of all ordinary processes of the body.
The nature of the biological clocks’ mechanism is still a mystery. Two quite different theories have been anvaned to account for them. According to the first of these theories, each individual contains its own independent timing system. This is believed to have evolved, aided by nature selection, as an adaptation to the rhythmic environment, it has now become independent of the environment. According to this view, the clocks are not perfect timers. They require regular of correction by the natural light and the changing lengths of the day throughout the year.
The other theory holds that living things react continuously to their rhythmic physical environment. The setting of their biological clocks, therefore, involves a constant adjustment to subtle environment forces. If this view is correct, the basic living clocks are potentially perfect timers.
Biological clocks appear to be everywhere in living things – even in individual cells or part of cells. But the search for the specific timing system has been futile thus far. Despite the careful study of many rhythmic phenomena and even of detailed chemical variations between cells, there is no evidence that any one of them is the clock –time itself. Not only has no independent timing system ever been discovered, but there has not yet been even a plausible guess as to its nature.