Circadian rhythms

Circadian rhythm is the name given to the roughly 24-hour cycles shown by physiological processes in plants, animals, fungi and cyanobacteria. (The term circadian comes from the Latin circa, "around", and dies, "day", meaning literally "around a day.") It was initially discovered in the movement of plant leaves in the 1700s by the French scientist Jean-Jacques d'Ortous de Mairan. The formal study of biological temporal rhythms (such as daily, weekly, seasonal, etc.) is called chronobiology.

The circadian rhythm is neither fully dependent on nor fully independent of external cues such as sunlight and temperature. Early researchers identified that some sort of "internal" rhythm must exist, because plants and animals did not react immediately to artificially-induced changes in daily rhythms. However it has been well established that a mechanism for adjustment also exists, as plants and animals will eventually adjust their internal clock to a new pattern (if it is sufficiently regular and not too far off the norm for the species). Overall, circadian rhythms are defined by three criteria:
 * 1) The rhythm persists in constant conditions (i.e., constant light) with a period of ~24 hours
 * 2) The rhythm period can be reset by exposure to a light or dark pulse
 * 3) The rhythm is temperature compensated, meaning it proceeds at the same rate within a range of temperatures.

Animal circadian rhythms
Circadian rhythms are important in determining the sleeping and feeding patterns of all animals, including humans. There are clear patterns of brain wave activity, hormone production, cell regeneration and other biological activities linked to this 24-hour cycle.

The circadian rhythm is linked to the light-dark cycle. Animals kept in total darkness for extended periods eventually function with a "free running" rhythm. Each "day," their sleep cycle is pushed back or forward (depending whether they are nocturnal or diurnal animals) by approximately one hour. Free-running rhythms of diurnal animals are close to 25 hours. Cues from the environment (Zeitgebers) ensure that the rhythms reset each day to the external rhythms. Free running organisms still have a consolidated sleep/wake cycle when in environment shielded from external cues, but the rhythm is not entrained and may become out of phase with other circadian, or ultradian rhythms (e.g., temperature and digestion). This research has influenced the design of spacecraft environments, as systems that mimic the light/dark cycle have been found to be highly beneficial to astronauts.

The circadian "clock" in mammals is located in the suprachiasmatic nucleus (SCN), a distinct group of cells located in the hypothalamus. Destruction of the SCN results in the complete absence of a regular sleep/wake rhythm. Contributing to this clock are photo receptors found in the retina, known as melanopsin ganglia. These cells, which contain a newly-discovered photo pigment known as melanopsin, follow a pathway called the retinohypothalamic tract, leading to the SCN. It is interesting to note that, if cells from the SCN are removed and cultured, they will maintain their own rhythm in the absence of external cues.

It appears that the SCN takes the information on day length from the retina, interprets it, and passes it on to the pineal gland (a pea-like structure found on the epithalamus), which then secretes the hormone melatonin in response. Secretion of melatonin peaks at night and ebbs during the day. The SCN does not appear to be able to react rapidly to changes in the light/dark cues.

Recently, evidence has emerged that circadian rhythms are found in many cells in the body--outside of the SCN "master clock." Liver cells, for example, appear to respond to feeding rather than light. Cells from many parts of the body appear to have "free-running" rhythms.

Disruption to rhythms usually have a negative effect in the short term. Many travelers have experienced the condition known as jet lag, with its associated symptoms of fatigue, disorientation and insomnia. A number of other sleep disorders are associated with irregular or pathological functioning of the circadian rhythms.

Recent research suggest that circadian rhythms and clock genes expressed in brain regions outside the SCN may significantly influence the effects produced by drugs of abuse such as cocaine. Moreover, genetic manipulations of clock genes profundly affect cocaine's actions.

Circadian rhythms also play a part in the reticular activating system in reticular formation.

Plant circadian rhythms
Plants are sessile organisms, and thus they are intimately associated with their environment. This ability to synchronize with daily changes in temperature and light is of great advantage to plants. For example, the circadian clock makes an essential contribution to photosynthesis, with the outcome that the clock is believed to increase plant growth and survival. As days grow shorter and cooler, plants are able to change the expression of their genes to prepare for the end of the growing season and to prepare for winter. At the most fundamental level, circadian rhythms are the cyclical expression of genes in individual cells. This cyclical expression is controlled by a central clock, which responds to light as well as temperature inputs.

The study of circadian rhythms is therefore of particular interest for plant biologists. Many of the circadian-controlled genes are involved in chilling and freezing tolerance, and photosynthesis. A better understanding of these genes could allow the creation of stress-tolerant plants, that are better able to survive in cold temperatures and grow with increased vigour. This will allow the expansion of both growing seasons and the growth range for many economically important crops.

Origin
Circadian rhythms are believed to have originated in the earliest cells to provide protection for replicating DNA, from high ultraviolet radiation during day-time. As a result, replication was relegated to the dark. The fungus Neurospora, which exists today, retains this clock-regulated mechanism.