Introduction

As a consequence of the Earth's daily rotation about its axis, all terrestrial organisms have evolved in an environment with alternating cycles of light and dark. During the course of evolution, organisms have adapted to the challenge of living with a light-dark

Figure 2 The important parameters of biological rhythms, zeitgebers, and reference time scales. Top panel, rhythm of a biological variable, x; bottom panel, cycle of an environmental zeitgeber, Z, which entrains the rhythm. These variables are plotted against two different time scales. The local time scale indicates the time of day at which the biological measurement was made. The circadian time scale standardizes the relationship of the biological variable to the zeitgeber cycle by defining circadian time 0 as dawn. The parameters for the zeitgeber are given in captial letters and those for the rhythm in lower case. Identified for rhythm and zeitgeber cycles are the mean values of the variable (x and Z), the maximum and minimum values (xmax, xmin, Zmax, Zmin), the ranges of the oscillations (r and R), the periods of the oscillations (t and T), the amplitudes (amax, amin, Amax, and Amin), and the reference phases (f and F). The figure also shows the phase relationship (C) between dawn on the zeitgeber cycle and the maximum of the rhythm, but C can be defined between any two reference points on the zeitgeber and rhythm waveforms. [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 2 The important parameters of biological rhythms, zeitgebers, and reference time scales. Top panel, rhythm of a biological variable, x; bottom panel, cycle of an environmental zeitgeber, Z, which entrains the rhythm. These variables are plotted against two different time scales. The local time scale indicates the time of day at which the biological measurement was made. The circadian time scale standardizes the relationship of the biological variable to the zeitgeber cycle by defining circadian time 0 as dawn. The parameters for the zeitgeber are given in captial letters and those for the rhythm in lower case. Identified for rhythm and zeitgeber cycles are the mean values of the variable (x and Z), the maximum and minimum values (xmax, xmin, Zmax, Zmin), the ranges of the oscillations (r and R), the periods of the oscillations (t and T), the amplitudes (amax, amin, Amax, and Amin), and the reference phases (f and F). The figure also shows the phase relationship (C) between dawn on the zeitgeber cycle and the maximum of the rhythm, but C can be defined between any two reference points on the zeitgeber and rhythm waveforms. [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].

(LD) cycle by occupying temporal niches in which they can optimize life-sustaining activities such as feeding, sleeping, and avoiding predators. It then is not surprising, perhaps, that organisms, ranging from prokaryotes to multicellular eukaryotes, have evolved an internal time-keeping system. In mammals, this internal time-keeping system, the circadian timing system (CTS), provides temporal organization for a host of behavioral, physiological, and biochemical variables. The fundamental adaptive advantage of this temporal organization is that it allows for predictive, rather than entirely reactive, homeostatic regulation of function. For example, prior to waking, body temperature, heart rate, blood pressure, and plasma cortisol increase in anticipation of increased energetic demands. In addition, the CTS, by monitoring day length and its rate and direction of change, contributes to adaptive reproductive and seasonal rhythms.

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