Biological rhythm
Biological rhythms are repetitive biological processes. Some types of biological rhythms have been described as biological clocks. They can range in frequency from microseconds to less than one repetitive event per decade. Biological rhythms are studied by chronobiology. In the biochemical context biological rhythms are called biochemical oscillations. The variations of the timing and duration of biological activity in living organisms occur for many essential biological processes. These occur (a) in animals (eating, sleeping, mating, hibernating, migration, cellular regeneration, etc.), (b) in plants (leaf movements, photosynthetic reactions, etc.), and in microbial organisms such as fungi and protozoa. They  have even been found in bacteria, especially among the cyanobacteria (aka blue-green algae, see bacterial circadian rhythms). Circadian rhythm. The best studied rhythm in chronobiology is the circadian rhythm, a roughly 24-hour cycle shown by physiological processes in all these organisms. The term "circadian" comes from the Latin "circa", meaning "around" and "dies", "day", meaning "approximately a day." It is regulated by circadian clocks. The circadian rhythm can further be broken down into routine cycles during the 24-hour day: While circadian rhythms are defined as regulated by endogenous processes, other biological cycles may be regulated by exogenous  signals. In some cases, multi-trophic systems may exhibit rhythms driven by the circadian clock of one of the members (which may also be influenced or reset by external factors). The endogenous plant cycles may regulate the activity of the bacterium by controlling availability of plant-produced photosynthate. Other cycles. Many other important cycles are also studied, including: Within each cycle, the time period during which the process is more active is called the "acrophase". When the process is less active, the cycle is in its "bathyphase" or "trough" phase. The particular moment of highest activity is the "peak" or  maximum; the lowest point is the "nadir". How high (or low) the process gets is measured by the "amplitude". Biochemical basis of biological rhythms. Goldbeter's book provides a thorough analysis of the biochemical mechanisms and their kinetic properties that underlie biological rhythms.