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circadian rhythm A term coined by Franz Halberg in 1959 to describe the approximately 24-hr biological cycles that are endo-genously generated by an organism (Latin: circa=about, dies=day).

entrainment When an exogenous stimulus achieves both phase and period control of one or more circadian oscillators, entrainment has occurred.

free-running rhythms Rhythms that persist even when an animal is isolated from external time cues. These rhythms do not damp out, are self-sustained, and have a period that is close to 24 hr.

masking Light and other stimuli can superimpose direct effects on the level of a variable that can alter the amplitude and waveform of circadian rhythms. These exogenous influences are referred to as "masking effects.'' Masking may cause an observed rhythm to inaccurately reflect the underlying circadian pacemaker.

phase The instantaneous state of an oscillation (i.e., maximum or minimum) defined by the value of a rhythmic variable and its time derivative. Often, in order to facilitate comparisons between rhythms or between rhythms and cyclic time cues, the time of a specific phase is chosen as a cycle reference point.

phase angle The difference in time between the phases of a rhythm and the entraining cycle or between two different rhythms. The phase angle is usually measured in fractions of the entire period.

phase response curve (PRC) A plot of the magnitude and direction of a phase shift as a function of the circadian phase at which a phase-shifting stimulus (typically light) is applied.

zeitgeber German for "time giver;'' an environmental time cue such as sunlight, food, noise, or social interaction that is capable of entraining the biological clock to a 24-hr day.

Biological rhythms are oscillations in an organism's physiology and behavior. These rhythms span a wide range of frequencies, ranging from fractions of a second to a year. Many rhythms exhibit a periodicity close to that of the 24-hr geophysical cycle that results from the daily rotation of the earth around its axis. Biochemical, physiological, and behavioral rhythms that oscillate with a period close to 24 hr are termed circadian rhythms. Circadian rhythms are generated by an endogenous pacemaker, are self-sustaining, and persist in the absence of time cues. It has been hypothesized that the original adaptive role for circadian organization was to ensure that phases of DNA replication sensitive to damage by high ultraviolet light levels in the primitive day were protected by being synchronized to night. Examples of human circadian rhythms are shown in Fig. 1. These include the daily oscillations of the sleep-wake cycle, body temperature, growth hormone, cortisol, and urinary potassium excretion. Circadian rhythms provide temporal organization and coordination for physiological, biochemical, and behavioral variables in all eukaryotic organisms and some prokaryotes. Without this important temporal framework, an organism will experience, minimally, a severely reduced homeostatic capacity. Circadian desynchronization can be likened to a symphony without a conductor—each instrument section may be fully functional, but the sections collectively are unable to harmonize. The medical importance of circadian rhythms is just beginning to be appreciated in the clinical arena. For example,

Encyclopedia of the Human Brain Volume 1

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Figure 1 A representative panel of human arcadian rhythms. Starting from the top panel: Rhythms in sleep, body temperature, plasma growth hormone, plasma cortisol, and urinary potassium excretion. These data represent 48 hr of data from a subject entrained to a 24-hr day (LD 16:8; 16 hr of light and 8 hr of dark). The light and dark periods are shown at the top of the data panels. [Reproduced by permission from Moore-Ede, M. C., Sulzman, F. M., and Fuller, C. A. (1982). The Clocks That Time Us: Physiology of the Circadian Timing System. Harvard University Press, Cambridge, MA].

Figure 1 A representative panel of human arcadian rhythms. Starting from the top panel: Rhythms in sleep, body temperature, plasma growth hormone, plasma cortisol, and urinary potassium excretion. These data represent 48 hr of data from a subject entrained to a 24-hr day (LD 16:8; 16 hr of light and 8 hr of dark). The light and dark periods are shown at the top of the data panels. [Reproduced by permission from Moore-Ede, M. C., Sulzman, F. M., and Fuller, C. A. (1982). The Clocks That Time Us: Physiology of the Circadian Timing System. Harvard University Press, Cambridge, MA].

circadian rhythms are now recognized as important variables in the diagnosis and treatment of many pathophysiological conditions. Many of the properties of circadian rhythms are analogous to physical oscillators, and, consequently, much of the terminology used to describe rhythms is derived from oscillator theory. All biological rhythms are characterized by fundamental parameters, which include period length, rhythm mean, circadian waveform, and circadian amplitude. Each parameter describes an important property of the circadian timing system. Figure 2 illustrates the important parameters of biological rhythms, zeitgebers, and reference time scales. After decades of circadian rhythm research, several generalizations can be made about circadian rhythms: (1) circadian rhythms are ubiquitous, they are present in all eukaryotes and some prokaryotes; (2) circadian rhythms are genetically determined, not learned; (3) circadian rhythms are generated by an endogenous, self-sustained pacemaker; (4) in an environment without external time cues, circadian rhythms persist with a period that approximates 24 hr; (5) the period of the circadian pacemaker is temperature-compensated; and (6) the circadian pacemaker can be entrained to external time cues.

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