biological time

Some biological patterns are cyclical and function as clocks, subject to resetting and breakdown. The Circadian rhythms ( the term means approximately daily and was coined by Franz Halberg in 1951) are an example of biological cycles which are non-linear oscillators, which mesh with day/night cycles. They are subject to entrainment or synchronization because of their time-dependent sensitivity. Exposed to some standard disturbance beginning at different times in the cycle, there will be different phase shifts inflicted. (see Winfree, 1987. p. 12) There is a particular point of vulnerability, where circadian rhythms can break down or become unpredictable when subject to a particular stimulus known as the "critical annihilating stimulus". This arrhythmic center in the pattern of timing is called its "phase singularity

Artur T. Winfree has studied the "geometry of biological time" using topology and phase portraits. He gives the example of the time zones of the world as an example. The "Isochronous" time lines divide up the equator. But what time is it at the South Pole? The lines all converge, and the sun actually appears to revolve around the horizon at a fixed height. This singularity is inherent in the geometry of the sphere and this mode of partitioning. Winfree uses the color wheel, where isochrones become isochromes as another example of the geometric requirement for singularity. If the color wheel runs around the circumference of a circle, there must be a place inside without hue. (This is grey for painters) Another topological example of this is the soap film around a ring. A hole must be made in the center for the soap to move to the edges. 

The heart is, of course, the most obvious example of a biological oscillator or clock. It too, can be knocked out of phase and into fibrillation, leading to death. (Fibrillation is comparable to turbulence in fluid dynamics. Every fiber of the heart is still beating, in fact severalfold more often than before. But the myriad muscular fibers have lost their simple coordination. They have become spatially decorrelated) It is also theorized that some chaotic element to the heart's normal functions enable it to reestablish regularity in the face of most disturbances. 

The heart's "vulnerable phase" is just as it is relaxing after a spontaneous contraction, (during 30 milliseconds in the electrocardiogram's T wave)