Chronobiology is that branch of science which objectively quantifies and explores mechanisms of biological time structure. It is an integrating discipline that impacts on all forms of life. When physiological functions are plotted along a time scale, they appear as regularly repetitive wave forms with means, amplitudes, phasing and periods. In nature these rhythms are found to have many frequencies, from a fraction of a second (ultradian) to a year or more (infradian or circannual); and those with periods of about one day (circadian) have been explored extensively. Examples of several circadian rhythms are given for experimental animals and man. Evidence is presented to show that it is particularly important to consider biological rhythmicity when interpreting experimental results or attempting to extrapolate from one species to another. An organism is indeed a different biochemical and morphological entity at different times, and it may be expected to react differently to a stimulus at different circadian phases. By taking advantage of natural rhythms in the susceptability to drugs, it is possible to optimize chemotherapy and radiotherapy for cancer and other diseases.

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