All complicated systems tend to oscillate.
The period lengths is the result of the properties of the system. It does not correspond to the period length of a rhythmic event in the physical environment. Such system dependent oscillations are foundfor instance in machines and bridges, but also in organisms. The speciality of biological systems is, however, that they have adapted themself during the course of evolution not only to the spatial structure of the surrounding, but also to its time structure. Corresponding rhythms are found in numerous organisms.