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Chronobiology 

Overview of human "circadian biological clock " with some physiological parameters.

Chronobiology is a field of   biology that examines periodic (cyclic) phenomena in living organisms and

their adaptation to solar - and lunar -related rhythms.[1] These cycles are known as biological rhythms.

Chronobiology comes from the ancient Greek  χρόνος (chrónos, meaning "time"), and  biology, which

 pertains to the study, or science, of  life. The related terms chronomics and chronome have been used in

some cases to describe either the molecular  mechanisms involved in chronobiological phenomena or themore quantitative aspects of chronobiology, particularly where comparison of cycles between organisms is

required.

Chronobiological studies include but are not limited to comparative anatomy,  physiology, genetics, 

molecular biology and  behavior  of organisms within biological rhythms mechanics.[1] Other aspects include

development, reproduction, ecology and evolution.

Description

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). The most important 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:[2] 

  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: white-

tailed deer, some bats)

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While circadian rhythms are defined as endogenously regulated, 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). For example, non-

symbiotic nitrogen-fixing bacteria diazotrophs growing in the rhizosphere of the rice  plant have been

shown to exhibit diurnal and seasonal variation in the rate of nitrogen fixation that is apparently regulated

 by cycles in the plant with a diurnal period mimicking (though lagging behind) the light-dark cycle.[3] In

this case, the endogenous plant cycles may regulate the activity of the bacterium by controlling availability

of plant-produced photosynthate.

Many other important cycles are also studied, including:

  Infradian rhythms, which are cycles longer than a day, such as the annual migration or reproduction

cycles found in certain animals or the human menstrual cycle. 

  Ultradian rhythms, 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.

  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.

  Gene oscillations  – some genes are expressed more during certain hours of the day than duringother hours.

Within each cycle, the time period during which the process is more active is called the  acrophase.[4] 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. 

History

A circadian cycle was first observed in the 18th century in the movement of plant leaves by the Frenchscientist Jean-Jacques d'Ortous de Mairan (for a description of circadian rhythms in plants by de Mairan,Linnaeus, and Darwin see this page). In 1751 Swedish  botanist and naturalist Carolus Linnaeus (Carl von

Linné) designed a floral clock  using certain species of  flowering plants. By arranging the selected species

in a circular pattern, he designed a clock that indicated the time of day by the flowers that were open at

each given hour. For example, among members of the daisy family, he used the hawk's beard  plant which

opened its flowers at 6:30 am and the hawkbit which did not open its flowers until 7 am.

The 1960 symposium at Cold Spring Harbor Laboratory laid the groundwork for the field of 

chronobiology.[5] 

It was also in 1960 that Patricia DeCoursey invented the  phase response curve, one of the major tools usedin the field since.

Franz Halberg of the University of Minnesota, who coined the word circadian, is widely considered the

"father of American chronobiology." However, it was Colin Pittendrigh and not Halberg who was elected

to lead the Society for Research in Biological Rhythms in the 1970s. Halberg wanted more emphasis on the

human and medical issues while Pittendrigh had his background more in evolution and ecology. With

Pittendrigh as leader, the Society members did basic research on all types of organisms, plants as well as

animals. More recently it has been difficult to get funding for such research on any other organisms than

mice, rats, humans[6][7] and fruit flies. 

Recent developments

More recently, light therapy and melatonin administration have been explored by Dr. Alfred J. Lewy 

(OHSU), Dr. Josephine Arendt (University of Surrey, UK) and other researchers as a means to reset animal

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and human circadian rhythms. Additionally, the presence of low-level light at night accelerates circadian

re-entrainment of hamsters of all ages by 50%; this is thought to be related to simulation of moonlight.[8] 

Humans can be morning people or evening people; these variations are called chronotypes for which there

are various assessment tools and biological markers.

In the second half of 20th century, substantial contributions and formalizations have been made by

Europeans such as Jürgen Aschoff  and Colin Pittendrigh, who pursued different but complementary views

on the phenomenon of  entrainment of the circadian system by light (parametric, continuous, tonic, gradualvs. nonparametric, discrete, phasic, instantaneous, respectively; see this historical article, subscription

required).

There is also a food-entrainable biological clock, which is not confined to the suprachiasmatic nucleus. The

location of this clock has been disputed. Working with mice, however, Fuller et al. concluded that the food-

entrainable clock seems to be located in the dorsomedial hypothalamus. During restricted feeding, it takes

over control of such functions as activity timing, increasing the chances of the animal successfully locating

food resources.[9] 

Other fields

Chronobiology is an interdisciplinary field of investigation. It interacts with medical and other research

fields such as sleep medicine, endocrinology, geriatrics, sports medicine, space medicine and

 photoperiodism.[10][11][12] 

The notion of   biorhythms, a classic example of   pseudoscience, which attempts to describe a set of cyclic

variations in human behavior based on physiological and emotional cycles, is not a part of 

chronobiology.[13]