**3. Distribution of biological rhythms**

**Exogenous rhythms**: exogenous rhythms are oscillations of the passive system that depend on periodic stimulus from the external environment to which the organism synchronizes with these rhythmical changes. They are only observed within the circadian periodicity of the social or climatic environment.

**5**

*Introductory Chapter: Chronobiology - The Science of Biological Time Structure*

**Endogenous rhythms**: endogenous rhythms are independently oscillating systems that are able to maintain their periodicity and also under constant nonperiodic

*Circadian rhythms*: oscillate with a period of approximately 24 ± 4 h. They are one of the most frequently followed and studied rhythms. The term "circadian" was first used by professor [1] and has essentially two meanings. The first describes the day and night part of the day as a whole and the second as a cycle with a period of

*Ultradian rhythms*: oscillate with a period <20 h. The frequency of ultradian rhythms varies considerably from one species to another and from one parameter to another. In humans, several functions oscillate in 60–120 min intervals, and these rhythms are sometimes superimposed on other functions that oscillate at 3–5 min intervals. *Infradian rhythms*: oscillate with the period >28 h. This term includes:

• *Circaseptan rhythms*: oscillate with a period of approximately 7 ± 3 days.

• *Circannual rhythms*: oscillate with a period of approximately 1 year

activity and the menstrual cycle in adult women.

they do not exist to support species preservation.

**4. Control of biological rhythms**

the timing of external cues.

SCN and peripheral oscillators.

nize from the SCN.

• *Circalunar rhythms*: rhythms with a period about 30 ± 5 days. Includes ovarial

(±2 months), synchronized or desynchronized within a calendar year. It also includes *seasonal rhythms* that are the result of an adaptation process of living organisms to the environment. In certain species, reproductive functions are stimulated at specific moments in the annual cycle, thus optimizing the survival of the species. Although seasonal rhythms are also observed in humans,

The control of circadian rhythms occurs at the level of the retina (light input), the suprachiasmatic nuclei of the hypothalamus (clock genes), and the pineal gland (melatonin synthesis). Daylight or equivalent simulated light impacts retinal cells and passes the retinohypothalamic tract into the hypothalamic suprachiasmatic nuclei, which are referred to as the "internal clock." The current hypothesis regarding the multioscillatory structure of the circadian system [2] contains the following components [3]:

**Inputs**: environmental periodic cues can reset the phase of the central pacemaker so that the period and phase of circadian rhythms become coincident with

**Central pacemakers**: the suprachiasmatic nucleus or nuclei (SCN) is considered to be the major pacemaker of the circadian system, driving circadian rhythmicity in other brain areas and peripheral tissues by sending them neural and humoral signals. **Peripheral oscillators**: most peripheral tissues and organs contain circadian oscillators. Usually they are under the control of the SCN; however, under some circumstances (e.g., restricted feeding, jet lag, and shift work), they can desynchro-

**Outputs**: central pacemakers and peripheral oscillators are responsible for the daily rhythmicity observed in most physiological and behavioral functions. Some of these over-rhythms (physical exercise, core temperature, sleep-wake cycle, and feeding time), in turn, provide feedback, which can modify the function of the

*DOI: http://dx.doi.org/10.5772/intechopen.88583*

conditions. Endogenous rhythms include:

approximately 24 h.

*Introductory Chapter: Chronobiology - The Science of Biological Time Structure DOI: http://dx.doi.org/10.5772/intechopen.88583*

**Endogenous rhythms**: endogenous rhythms are independently oscillating systems that are able to maintain their periodicity and also under constant nonperiodic conditions. Endogenous rhythms include:

*Circadian rhythms*: oscillate with a period of approximately 24 ± 4 h. They are one of the most frequently followed and studied rhythms. The term "circadian" was first used by professor [1] and has essentially two meanings. The first describes the day and night part of the day as a whole and the second as a cycle with a period of approximately 24 h.

*Ultradian rhythms*: oscillate with a period <20 h. The frequency of ultradian rhythms varies considerably from one species to another and from one parameter to another. In humans, several functions oscillate in 60–120 min intervals, and these rhythms are sometimes superimposed on other functions that oscillate at 3–5 min intervals.

*Infradian rhythms*: oscillate with the period >28 h. This term includes:


#### **4. Control of biological rhythms**

The control of circadian rhythms occurs at the level of the retina (light input), the suprachiasmatic nuclei of the hypothalamus (clock genes), and the pineal gland (melatonin synthesis). Daylight or equivalent simulated light impacts retinal cells and passes the retinohypothalamic tract into the hypothalamic suprachiasmatic nuclei, which are referred to as the "internal clock." The current hypothesis regarding the multioscillatory structure of the circadian system [2] contains the following components [3]:

**Inputs**: environmental periodic cues can reset the phase of the central pacemaker so that the period and phase of circadian rhythms become coincident with the timing of external cues.

**Central pacemakers**: the suprachiasmatic nucleus or nuclei (SCN) is considered to be the major pacemaker of the circadian system, driving circadian rhythmicity in other brain areas and peripheral tissues by sending them neural and humoral signals.

**Peripheral oscillators**: most peripheral tissues and organs contain circadian oscillators. Usually they are under the control of the SCN; however, under some circumstances (e.g., restricted feeding, jet lag, and shift work), they can desynchronize from the SCN.

**Outputs**: central pacemakers and peripheral oscillators are responsible for the daily rhythmicity observed in most physiological and behavioral functions. Some of these over-rhythms (physical exercise, core temperature, sleep-wake cycle, and feeding time), in turn, provide feedback, which can modify the function of the SCN and peripheral oscillators.

*Chronobiology - The Science of Biological Time Structure*

time dependence on the following:

• Results of diagnostic tests

• Pharmacokinetics of drugs

effect—drug properties

tions to maintain these constant conditions.

**3. Distribution of biological rhythms**

• Sensitivity to drugs

• Occurrence or worsening of disease manifestations

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

• Sensitivity to drugs in terms of pharmacodynamic—either therapeutic or side

**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 func-

**Exogenous rhythms**: exogenous rhythms are oscillations of the passive system

that depend on periodic stimulus from the external environment to which the organism synchronizes with these rhythmical changes. They are only observed

within the circadian periodicity of the social or climatic environment.

**4**
