Rhythmic variations with frequences from fractions of seconds to years characterise a wide variety of biological processes (Aschoff, 1979). Biological rhythms can be observed, not only in the individual of the species, but also in the cells which comprise the organism and the populations of which it is a member. These regular fluctuations can be endogenously generated by some form of internal oscillator, or alternatively may passively reflect exogenous environmental alterations. An important group of rhythms combines both endogenous and exogenous inputs with an internal oscillator or oscillators which are capable of being influenced by some external change. In this situation, the internal rhythm is kept in harmony with an environmental cycle by a change in the outside world acting as a synchroniser or zeitgeber. In this type if the animal is artificially isolated from its normal external synchroniser, the rhythm will continue, but free running, with a periodicity which is a close approximation to the duration of the environmental cycle to which it is normally tied. These rhythms normally synchronised to an environmental cycle but capable of being self-sustaining at approximately the same rate, are termed circa rhythms: thus circadian, circannual and circalunar rhythms, according to the geophysical cycle by which they are normally entrained.