6.4.2.1. DEFINITIONS*

Vital activities which characterize cells, tissues, organs, individuals and even populations, do not occur evenly or randomly in time, but have a daily maximum and minimum.
A rhythm can be defined as being the periodic recurrence of a sequence of events occurring in the same order during the same time interval.
Four parameters characterize a biological rhythm: period, mesor, amplitude, acrophase.
 

 
A rhythm is called ultradian, circadian or infradian when the period is lower than, equal to or greater than 24 hours respectively.
6.4.2.2. TEMPORAL STRUCTURE

Peaks of the different variables are not randomly distributed. On the contrary, a time-related or "temporal structure" organization exists for the distribution of the acrophases over 24-hour periods.
The Various vital functions have strong phase relations between the ascending and descending parts of their rhythms.
The alteration of these phase relations is at the origin of the disorders related to jet lag and shift work.
Chronobiology is defined as being "the study of the temporal structure of each living being and that of its alterations".
 
6.4.2.3. ORIGIN

Circadian rhythm periods are close to those of the earth's rotation.
One is therefore led to think that these rhythms strictly and rigorously depend on environmental variation, i.e. day-night alternation, darkness-daylight, heat-cold, rest-activity, etc.
Under constant environmental and behavioral conditions (experiment called "constant routine") biological rhythms do persist.
We are therefore left to conclude that an internal mechanism regulates body function. This mechanism is simply known as the biological clock.
 
6.4.2.4. IMPLICATIONS

In certain cases, choosing the best time to administer a drug allows its therapeutical effects to be optimized and its secondary effects reduced. For example, as the acrophase of cortisol secretion is located around 8am, administering corticoids (drugs chemically similar to cortisol) in phase with its peak secretion reinforces their actions. Another example is the time duration over which an antihistamine drug will act; the effective time is doubled if the drug is administered at 7am rather than 7pm.
The effects of various toxic agents (heavy metals, organic solvents, carbon oxides) vary considerably over a period of 24 hours. At certain times, a toxic agent can cause substantial lesions, whereas the same dose received at another time in the 24-hour cycle would be much less harmful. These temporal differences in the intensity of toxic effects are most often correlated with the capacity of the liver and the kidneys to "detoxicate" blood of these harmful substances.
6.4.3. CIRCADIAN RHYTHMS OF PERFORMANCE

6.4.3.1. DESCRIPTION

Operators engaged in any given tasks do not always react in the same way at different times of day and night. Similar to biological functions, perceptual and mental functions are subject to rhythmic variations over a 24-hour span.
　
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