Ever since publication of the challenging scientific article entitled Biological clock in the unicorn (Cole, 1957), research concerning the rhythmic behaviour and physiology of plants and animals has overwhelmingly supported the view that such processes are not controlled solely by cyclical changes in the environment, however subtle some of those variables might be. It has long been apparent that biorhythms can be expressed spontaneously, at periodicities somewhat different from those of geophysical variables, in isolation from the environment, leading to the conclusion that some form of internal biological clockwork is involved in their timing. It has been questioned as to how this free-running ability has provided adaptive advantage during the evolutionary process (Winfree, 1987) but, as discussed in Chapter 3, it seems to follow logically if, as appears to be the case, endogenous rhythmicity provides an organism with anticipatory capability in relation to potentially adverse or favourable phases of their cyclical environment. For example, for photosynthetic unicellular organisms early in evolution, anticipatory circadian physiology would permit the capture of the first photons at dawn (Suzuki and Johnson, 2001), would permit favourable phasing of incompatible processes such as photosynthesis and nitrogen fixation (Mitsui et al., 1986) and permit favourable phasing of cellular events that are inhibited by sunlight (Pittendrigh, 1993).