Many natural biological processes are cycles; this means that different state of the body alternate with sufficiently clear frequency. The simple examples of quick cycles are breathing or heartbeating when the periods of muscle contraction are alternated with the periods of muscle relaxation. The other vital cycle, such as the alternation of wakefulness and sleeping, are the short-term periods with approximately 24 hours cycle. If a biological cycle is a tides dependent, they are called tidal, it has cycle of 12.4 or 24.8 hours longevity. The period of lunar biological cycles corresponds to the length of the lunar month.
Biological cycles are interesting because in many cases they are independent from ambient conditions. Such cycles are called endogenous, i.e., "coming from the inside": though usually they are correlated with the rhythmic changes in the external environment, such as the alternation of day and night, they cannot be considered as a direct response to these changes.
Endogenous biological cycles occur in all organisms except bacteria. The internal mechanism that supports the endogenous cycling is called the biological clock.
The biological clock functioning is now well understood, but its underlying processes remain a mystery. In the 1950's, the Russian chemist Belousov proved that even in a homogeneous mixture, some chemical reactions can speed up and slow down from time to time. Similarly, ethanol fermentation in yeast cells is activated and inhibited with the period of 30 seconds. Somehow these cells communicate with each other, so their rhythms are synchronized and all yeast suspension is "pulsing" twice per minute.
Circadian cycles with a period of about 24 hours are of great interest. They have gotten their name from the Lat. Circa - about and dies — day.
Biological processes with circadian periodicity are very diverse. For example, three kinds of glowing mushrooms strengthen and weaken their glow every 24 hours, even if artificially kept under constant light or in complete darkness. Various metabolic processes in higher plants, such as photosynthesis and respiration, occur with circadian cycle. A variety of circadian cycles is also known in animals.
Biological clock mechanism is very precise in insects. For example, the bees know when certain flowers are open, and visit them every day at the same time. Bees also quickly learn what time they are given sugar syrup at the apiary.
In human body not only sleeping, but also many other functions are subject to a daily rhythm.
Examples are raising and lowering blood pressure and the allocation of the potassium and sodium emission by kidneys, fluctuations in reflex time, sweating, etc. Changes in body temperature are especially noticeable: at night, it is by about 1°C lower than in the afternoon.
Biological cycles in humans are formed gradually in the course of individual development. A newborn has quite unstable periods of sleep, nutrition, etc. Regular change of periods of sleep and wakefulness, based on the 24-25 hour cycle starts with only at 15 weeks of age.
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