The spectrum of biological rhythms exhibits characteristic principles of biological time structure which also rule the functional behaviour. With increasing period lengths the rhythms become increasingly complex. In the long-wave section the rhythmic functions find their corresponding cycles in the environment, whereas the shorter waves represent only endogenous autonomous rhythms, which maintain an internal time order by means of frequency- and phase-coordination. Under resting conditions and in a state of complete adaptation only a few spontaneous rhythms dominate in the spectrum. However, under loading conditions as well as in pathological situations further periodicities come up. The spectrum of rhythms can be divided into certain blocks, with the period lengths predominating in each of these whole number frequency ratios forming a harmonic system. Frequency- and phase coordination establish a system of co-action which favours the functional economy of the organism. A tripartite organization of the autonomous rhythms involves different functional behaviours with regard to frequency, amplitude, and phase. Slower rhythms act upon the faster rhythms preferably by modulating their frequencies, while changes of the faster rhythms influence the slower ones by enhancing their amplitudes, multiplying their period lengths and shifting their phases. In principle the reactions of living systems are periodically structured. Reactive periodicity brings to appearance an endogenous time structure, which prefers whole number relationships with the spontaneous rhythms. The phase position of reactive periods depends on the stimulus. The amplitudes dampen down with increasing compensation. From the medical point of view so-called circaseptan (about 7 days) reactive periods are of predominant interest. This periodicity can be observed in numerous adaptive and compensating processes. It does not depend on the external week cycle and was already known to the antiquity.