Circadian Rhythms

    Circadian Rhythm is a chronobiological term defined as a cyclical variation in the intensity of a metabolic or physiological process, or of some facet of behavior, with a period of about 24 hours when in constant conditions. The term “circadian” is derived from the Latin for “about a day”.
    The exogenous periodicity of the external environment which acts as ‘time-giver’ (or zeitgeber) in the form of light-dark, temperature and (possibly) nutrient cycles, can serve as a cue to organisms so that their endogenous biological clocks allow for the appropriate temporal synchronization of their physiology and behavior, optimizing their metabolic and physiological efficiency. Circadian clocks throughout phylogeny share fundamental properties, the two most important being temperature compensation, without which clock accuracy would suffer at the hands of fluctuations in ambient temperature, in both homeotherms (warm-blooded) and poikilotherms (cold-blooded), and entrainment to light–dark cycles, the process by which input from photoreceptors keeps circadian clocks set precisely to local time.

Fig.1: The graph above shows the periodic rhymicity of general physiological/behavioral activities based on 24-hr repeating rate, i.e. the activity is circadian, for a total time of 120 (24 x 5) hours. Figure made by Kamesh Regmi.

Biological Timekeeping Phenomena:

Many of the environmental pressures exerted on organisms arrive more or less randomly – sudden death, meeting a rival/predator, infection, cloud cover – but many more arrive quite as expected, like the appearance of daylight, high tide, full moon, onset of summer, and so on. The ability to anticipate these events in order to avoid their dangers and exploit their benefits is of great adaptive advantage to any organism and is inherited just as any other physiological characteristics. Hence, most organisms possess some biological machinery to measure the time-tabling of the environment. Most organisms have principally two different ways of measuring time: either they can rely on the exogenous periodicity of the environment, using that as their clock so that they synchronously get up at sunrise, hide under stones at low tide, and so on; or they can use some endogenous mechanism to measure time, rather in the sense that a man-made clock does.
    It is also worth mentioning that many rhythms that many organisms seem to follow with their internal biological clock, like tidal rhythms, sun-compass clocks, and photoperiodism are mostly based on the fundamental 24-hours (i.e. circadian) rhythm of organism. It thus appears that the environmental effects of the earth’s clockwork rotation about its axis have produced, by natural selection, an analogous/homologous internal clock of roughly 24-hour periodicity in animals (and plants). One of the primary features of the biological clocks is that even though they have, in all probability, evolved independently many times, their formal characteristics are similar in diverse organisms. Also, these timekeeping abilities are apparently almost always driven by an internal clock that “oscillates” (i.e. by one that repeats the same physiological/metabolic cycle approximately once per 24 hour) rather than by using some non-oscillatory mechanism such as a process that counts high frequency events (like heart beats; because they can be both internally and externally regulated).

Fig. 2: Schematic diagram of the general ways in which the synchrony between the external and the internal environment of the organism interact to keep the internal clock running with predominantly circadian periodicity. Figure made by Kamesh Regmi.

The ensuing pages focus on the FOUR COMMANDMENTS, and a somewhat detailed analysis of each commandment as it pertains to our topic - CIRCADIAN RHYTHMS.


Art: Kamesh Regmi.