Editing Chronobiology (section)

==The subject==
Chronobiology studies variations of the timing and duration of biological activity in living organisms which 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]]). 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 clock]]s.

The circadian rhythm regulates behaviour including timing of the activity phase. Depending on their innate active phase, organisms can be classified into one of three categories:<ref>Nelson RJ. 2005. An Introduction to Behavioral Endocrinology. Sinauer Associates, Inc.: Massachusetts. Pg587.</ref>
* [[Diurnality|Diurnal]], which describes organisms active during daytime
* [[Nocturnal]], which describes organisms active in the night
* [[Crepuscular]], which describes animals primarily active during the dawn and dusk hours (ex: domestic cats,<ref>{{Cite journal |last=Merčnik |first=Neva |last2=Prevolnik Povše |first2=Maja |last3=Škorjanc |first3=Dejan |last4=Skok |first4=Janko |date=2023-12-01 |title=Chronobiology of free-ranging domestic cats: Circadian, lunar and seasonal activity rhythms in a wildlife corridor |url=https://www.sciencedirect.com/science/article/pii/S0168159123002666 |journal=Applied Animal Behaviour Science |volume=269 |pages=106094 |doi=10.1016/j.applanim.2023.106094 |issn=0168-1591}}</ref> white-tailed deer, some bats)

While [[circadian rhythms]] are generated by [[endogenous]] processes, they can be regulated by both endogenous and exogenous signals. Other biological cycles may be regulated by exogenous signals. 

Many other important cycles are also studied, including:
* [[Infradian rhythm]]s, which are cycles longer than a day. Examples include [[Circannual Cycle|circannual]] or annual cycles that govern migration or reproduction cycles in many plants and animals, or the human [[menstrual cycle]].
* [[Ultradian rhythm]]s, which are cycles shorter than 24 hours, such as the 90-minute [[REM cycle]], the 4-hour [[nasal cycle]], or the 3-hour cycle of [[growth hormone]] production.{{Citation needed|date=November 2023}}
* [[Tide|Tidal rhythms]], commonly observed in marine life, which follow the roughly 12.4-hour transition from high to low tide and back.
* [[Lunar rhythms]], which follow the [[lunar month]] (29.5 days). They are relevant e.g. for marine life, as the level of the tides is modulated across the lunar cycle. 

Within each cycle, the time period during which the process is more active is called the ''[[wikt:acrophase|acrophase]]''.<ref>Refinetti, Roberto (2006). ''Circadian Physiology''. CRC Press/Taylor & Francis Group. {{ISBN|0-8493-2233-2}}.  [http://www.circadian.org/dictionary.html Lay summary]</ref>  When the process is less active, the cycle is in its ''[[wikt:bathyphase|bathyphase]]'' or ''trough'' phase.  The particular moment of highest activity is the ''peak'' or ''maximum''; the lowest point is the ''nadir''.