Biological Rhythms

Biological rhythms are oscillatory processes observed in living beings—while the beings are alive or even after their death, as is the case for populational oscillations determined by the study of geological cycles using fossilized organisms. Until life on other planets can be documented, biological rhythms are considered to be restricted to organisms on planet Earth. The most obvious separation between types of biological rhythms is the one between rhythms that are purely caused by environmental cycles on one hand and rhythms that are generated endogenously and are only modulated by environmental cycles on the other hand. Technically, the term “circa” should be used only for rhythms that have been demonstrated to be endogenously generated and to be susceptible to synchronization by environmental cycles, which as of today applies only to circatidal, circalunar, circadian, and circannual rhythms. However, there are many other oscillatory patterns that have been observed in living organisms, and they are undoubtedly biological rhythms even if many of them may not be controlled by biological clocks. Biological rhythms are usually classified into three main categories: those that oscillate about once a day (circadian), those that oscillate faster than once a day (ultradian), and those that oscillate slower than once a day (infradian). Bibliographies in these three categories will be detailed below, after a brief discussion of professional journals and considerations about the reliability of oscillations presumed to constitute biological rhythms.