In contrast, the mechanism that runs a circadian clock, as an ESSO, does not cease its time-keeping activity, and does not require an outside influence to continue keeping time.įigure 2.1: An hourglass and a common analog clock.

Similarly, an hourglass is not an oscillator: the sand only runs in one direction, and then time-keeping ceases. It must be manually turned over and reset – it requires an outside influence to count down another length of time. However, an hourglass is not a self-sustained method of keeping time: once the sand has run down, the hourglass cannot, on its own, count down another duration of time again. When an hourglass is turned over, the duration it takes for the sand to run down may be used to measure a determinate length of time. An hourglass or sand timer is a potentially very useful device for keeping track of time, but it is not an ESSO. It is useful to compare and contrast a biological ESSO with artificial time-keeping devices. An endogenous oscillator is one that is internal to the organism and not a reflection environmental inputs. (A heart would be considered self-sustained since it persists throughout the life-span). Unlike a pendulum clock which eventually runs down and needs to be started again, a self-sustained clock requires no external event for it to continue to oscillate. An oscillator is self-sustained if it can continue to count off these ~1-day-long periods indefinitely without damping out. Because we are dealing with circadian clocks, the period or length of time that it takes the clock to oscillate is about 24 hours (again: circa-dian, about a day). Two familiar examples are the swinging of a pendulum or pulsing of a beating heart. An oscillator is a mechanism that fluctuates between different states repeatedly and with a stable frequency. A Basic Sketch of Endogenous, Self-Sustained OscillatorsĪt the heart of a circadian clock is an endogenous, self-sustained oscillator (ESSO).

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