Within the broad range of physiological oscillations is a sub-group which occurs in physiological systems involved in homeostasis. Such systems have the potential for oscillatory behaviour because their distributed nature allows the existence of significant time delays in reflex arcs. Physiological systems of this type can be considered as a number of different elements linked together, typically by neural pathways, to give the negative feedback necessary for homeostasis. Often such systems behave as spontaneous oscillators (Hyndman, Kitney and Sayers, 1971), the frequency of oscillation being determined by the characteristics of the system. A change in these characteristics can be considered to be equivalent to changing the setting of an oscillator so that it takes up a new and normally stable oscillatory mode. It is possible to envisage the human body as containing a number of systems which frequently oscillate. There is also evidence that biological systems which oscillate at frequencies higher than that consistent with the 24-hour day interact with each other, e.g. the interaction between heart rate control and respiration.

Historically, there have been a number of different theories relating the nature of biological rhythms. The current consensus appears to be that biological systems which exhibit oscillations can be divided into autonomous or endogenous systems (i.e. systems which are self-sustaining), which usually oscillate at periods of less than 1 hour, and non-autonomous or exogenous systems (systems which require external stimulation).