**1. Introduction**

Both humans and most animal species on our planet are exposed to regular alternations of light and darkness, with a constant periodicity of 24 h throughout their lifetime. This regular alternation of light and dark affects not only human biological systems but also the social organization of behavior. Processes occurring in the human organism, which are dependent on the periodic alteration and alternation of environmental factors, potentially affect the blind and those employed in shift work and "nocturnal types," whose habits deviate from the usual mode of most individuals. The mode of alternating light and darkness is different at the equator, behind the polar circles, and also at transitions across multiple time zones. The effect of such light modes is manifested in periodic changes in several physiological functions and biological rhythms exhibited at every level of life—in single cells, tissues, organs, and, ultimately, physiological systems of organisms.

Changes in the external environment, such as those of the weather or atmospheric conditions, can be **unpredictable**; therefore, organisms need systems that directly respond to changing environments. However, there are also **predictable** changes, which are the result of specific planetary movements such as the day-night cycle (rotation of the earth on its axis), the cycle of the moon (cycle of the moon around the earth), or annual cycles (cycle of the earth around the sun). For these predicable changes, organisms have specific mechanisms that generate endogenous biological rhythms corresponding directly to certain periodicities in the environment. They are not directly dependent on the rhythmicities of environment but only use the periodic information from the environment to synchronize biological oscillations with cycles of environment.

#### **2. Biological rhythms and homeostasis**

Biological rhythms are sequences of events that are repeated over time in the same manner and with the same interval; in other words, they are predictable in time. Biorhythms of organisms are the result of adaptation to changes in the environment, which is highly variable and exhibits variation in many factors. Chronobiology is the science investigating and objectively quantifying the mechanisms of biological time structure, including the rhythmic manifestations of life.

Because medicine is, in large part, based on homeostatic principles, the following scheme may be regarded as representative.

The survival of the animal in a highly periodic "day-night" environment, therefore, depends on the approximate timing of its reactions. Physiological systems must integrate and subsequently influence the responses of each system to different times of the day. Therefore, the traditional concept of homeostasis began to change. The **old concept of homeostasis** states that physiological variables are relatively constant only in a narrow physiological range throughout the day, which is essential for health. All organs and tissues of the body perform functions to maintain these constant conditions. However, the traditional concept of homeostasis involves no time dependence on the following:


**In contrast, the new concept of homeostasis** states that physiological variables oscillate and are adjusted only to a narrow physiological range throughout the day, which is essential for health, and all organs and tissues of the body perform functions to maintain these constant conditions.
