Circadian rhythm

A circadian rhythm or circadian rhythm (Latin circum "around ... around round" and this " day " and Greek ῥυθμική rhythmiké or ῥυθμός rhythmos "rhythm" ) called in chronobiology endogenous ( internal ) rhythms having a period length of have approximately 24 hours. This term was introduced by Franz Halberg in 1959. He is now often Germanized written circadian rhythm. There are a plurality of biological circadian rhythms. However, they all ultimately an analogue of human sleep -wake cycle dar. Animals and plants have a sleep-wake rhythm. In the vernacular, is the circadian rhythm as the "internal clock " known.

Basic Properties

Although the biological background and the mechanisms of circadian rhythms between different organisms are different, have circadian rhythms certain characteristics that are common to many different types. The exact period length can vary between different species, but is usually 22 to 25 hours. The inner rhythm does not require signals from the outside world to follow his rhythm, which is not always exactly 24 hours. However, the process can be adapted to a precise 24 -hour cycle by, can be corrected with the help of external stimuli, the so-called timers. This process is called synchronization.

The external stimuli that can serve as a timer, are different for different species, but the most important and perhaps the most famous is the light. Another timer, for example temperature and social stimuli (e.g., alarm).

Another feature of the internal clocks that you previously did not quite, is that they are not affected by pH or by the body temperature of the organism, changes in the surrounding temperature can be a sign of morning or evening for some species, but although almost all known chemical reactions proceed more rapidly at higher temperatures, the periodicity in the organism of temperature and pH independent.

Period length ( τ ), phase ( Φ ) and Phasenrespons

A circadian rhythm is characterized by a specific period length, that is, each iteration takes time. The period length is often referred to with the Greek letters Tau ( τ ) and lasts for most organisms approximately 24 hours. If an organism is kept in a constant environment, ie with a constant amount of light and temperature around the clock, he will follow a daily cycle, the length of which depends on its internal clock. Over time, the internal clock can deviate from the true course of time more and more.

The period length of the internal clock depends on the genetic makeup, and it's possible to breed organisms that have an internal clock with a longer or shorter period length. You can also manipulate the τ of an organism with drugs or hormones or altered by manipulating the environment of the organism. The age of the organism also influences the period length of the circadian clock. In some organisms, such as humans, τ decreases with increasing age, while τ increases in other organisms, such as mice with age. It is also possible, by artificial light to change τ. Cockroaches, which are exposed in a 22 -hour cycle, developed a shorter cycle length than cockroaches were exposed to a 26 hour cycle. These effects persist long time, even after the experiment is completed.

The date according to the internal clock when the organism is " expected " that a particular event will occur (eg sunrise or sunset), phase is called and is denoted by the Greek letter Phi ( Φ ). Since the period length of the circadian clock is not exactly 24 hours and the time of sunrise and sunset varies throughout the year, the internal clock must be able to be corrected with the aid of external timing signals. For a large part of the subjective day (that is the time when the internal clock "believes" that it is day) will not cause light to a phase shift. This is shown in the Phasenresponskurve by values ​​close to zero. Although no significant phase shift is taking place, it seems that light middle of the day has a certain importance for the circadian rhythm. Light at subjective evening and in the subjective night will turn back the biological clock, while light "front" turns during the late subjective night, the internal clock after.

Strong and weak phase shift: Type -0 and Type -1 Respons

Phasenresponskurven can have two fundamentally different impressions, depending on the organism and the intensity of the timer (eg light intensity, if one uses light as a timer ). We distinguish between type 0 and type 1 Phasenresponskurven Phasenresponskurven. A type 1 Phasenresponskurve means that the response from the daily rhythm on a timer is relatively small ( the longest a few hours). An example of a type 1 Phasenresponskurve shown above. A type 0 Phasenresponskurve is characterized in that it somewhere in the cycle a " decisive moment ", when a timer can move up to 12 hours forward or back. Whether an organization with type -0 or type 1 reaction reacts the reaction depends on the nature of the organism, and the intensity of the stimulus. If the stimulus intensity is increased ( for example the intensity, if one uses light as a timer ), you can bring an organism that has a Type 1 Phasenresponskurve for light usually means that it reacts with a type 0 reaction. A study has shown that people who were consecutively exposed to three days in the morning bright light can react with a so-called strong phase shift, ie. Having a type -0 reaction

Phase angle ( ψ )

The subjective or circadian time ( ie the time that follows the internal clock of the organism without regard to the outside world ), may be more or less well adapted to the objective ( external ) time. The clearest example of this that the circadian and distinguish the objective time may, is perhaps the so-called jet lag. The difference between the objective and circadian time can either hours or a so-called phase angle, that is, into degrees can be expressed. The difference between circadian and objective time is denoted by the Greek letter psi ( ψ ). If we express ψ as degree angle, ψ represents a 180 ° difference between subjective and objective time of 12 hours.

Synchronization and timing

Light as a timer

Light is believed to be the timer whose effect is the most universally. It acts as a timer in almost all organisms studied, including those who live in perpetual darkness. The organism reacts to light in the area with a light-sensitive pigment that exists in the retina ( in vertebrates ) or in other cells ( in insects and plants) are either. Plants have three different classes of light-sensitive pigments in addition to chlorophyll, namely phytochromes, which are sensitive mainly to red light, but also to a lesser extent for blue light, cryptochromes, which are sensitive mainly to blue light, but also as signaling molecules used be if the phytochrome light "catch", also phototropins, which are not involved in the regulation of circadian rhythms, but also control the phototropism of plants, that is, that the plant accrues to a light source. The plant regulates its sensitivity to light through the production of phytochromes and cryptochromes. It is greatest in the morning and a few hours later. During this time the plant is most sensitive to light.

Function

The circadian rhythm helps an organism to adapt to daily recurring phenomena. It controls or influences, for example, leaf movements or flowers opening in plants and in animals, the heart rate, the sleep-wake rhythm, blood pressure and body temperature.

In addition to the endogenous nature of these rhythms is the free-running under constant conditions, the relative insensitivity to ambient temperature, the Entrainierbarkeit in a specific and limited time encoder range and a genetic disposition (see chronotype in humans) indicative of the circadian rhythms.

Since the external cause of the circadian rhythm is the self-rotation of our planet, serves as the most obvious external change of rhythm, the intensity of illumination of our atmosphere. This pacemaker is detected in the visual system, in some cases, the changing position of the sun.

The continued existence of a free-running circadian rhythm under constant conditions shows that there must be generating inner unity an oscillator, a rhythm. As long as it is not known how this oscillator is working, you can only on the perceived rhythm measurements run under as far as possible eliminate external rhythm generator. Properties of the oscillator must then be derived from the behavior: the classic " black box" - method of behavioral research as it specifically distinguishes the behaviorism. For some groups of animals now at least parts of the black box in the central nervous system (CNS) could be localized.

In all organisms studied cryptochrome seems to play a crucial role in the readjustment of the internal clock:

• Insects: in the optic lobes
• Molluscs: at the base of the retina
• Vertebrates: the presence, on the junction of the optic nerve, the suprachiasmatic nucleus ( SCN ) and / or in the pineal gland ( posterior pituitary or pineal gland, also called epiphysis ). The pineal gland produces important for sleep-wake rhythms hormone melatonin.

In fish, amphibians, reptiles and many birds, the pineal gland, however, is still sensitive to light. In some amphibians, a so-called vertex eye is observed: a skull opening, which is only covered by meninges and skin and thus transmits light into the brain ( " Third Eye "). It also controls in reptiles and some birds except the circadian production of melatonin circadian rhythms also other such as body temperature and food intake. One can assume that it is evolutionarily older than the suprachiasmatic nucleus ( SCN).

Molecular Biology

In mammals, the central circadian pacemaker place in the suprachiasmatic nucleus of the hypothalamus, the more peripheral pacemaker coordinated. The molecular clock runs by a transcription-translation feedback, by making protein translation inhibits the transcription of the gene of this protein. Participants include several proteins, one of which shall CLOCK, BMAL1, PER, CRY and NPAS2 as key proteins. The circadian molecular clock ( circadian molecular clock = CMC ) here has a positive arm with a CLOCK - BMAL1 heterodimer that stimulates the negative arm with a PER- CRY heterodimer, which inhibits the positive arm. A feedback sequence takes approximately 24 hours, with an oscillation of the protein expression is. This is controlled for the two proteins BMAL1 and CLOCK by two nuclear receptors ( REV - ERB - α and REV - ERB - β ) and thereby modulates the circadian rhythm. Peripheral tissues have a similar cycle, but are synchronized from the central pacemaker by indirect neural and hormonal signals and temperature changes.

The central pacemaker may be changed by external effects, such as light, these effects are referred to as " time encoder " in English. However, the adaptation needs depends on the change in the " time encoder " in some cases several days and can take different lengths of time in the peripheral pacemakers. This allows an internal dyssynchrony example are triggered by the jet lag.

The developed synthetic agonist SR9009 SR9011 and the nuclear receptors REV - ERB - α and REV - ERB - β can reduce the strength of the circadian Oszillationenn by inhibition of BMAL1 expression. In mice, the injection of the agonist resulted in an increased basal oxygen demand and loss of adipose tissue. Furthermore, a decreased lipogenesis in the liver, increased glucose and lipid oxidation in muscle and decreased triglyceride synthesis and storage in white fat cells showed.

Human chronotypes

In the population into two main categories can be distinguished from Chrono types. Some like to go to bed late and sleep like it - the "owls", while the " larks " go to bed early and get up early. These differences are most likely materialize by genetic predisposition. The cause of a different expression of the gene PER2 is discussed. In studies with young people, of which during puberty most as "owls" can be characterized, for example, could be demonstrated that a one hour delayed start of the day's activities - led to a general improvement in performance and better health - especially in winter.

Another chronobiological approach is the changing age structure of our society. In babies still outweighs the ultradian system - short periods of activity alternate with periods of short sleep partly on not even half an hour - to the rhythm of the infant is increasingly controlled by the circadian system. In old age, however, it loses again in influence.

People often live in contrast to their circadian rhythm. Thus, the proportion of to shift work. In addition, less time is spent in daylight, especially in the winter where the light rays indoors rarely higher than 500 lux. Even an overcast sky outdoors has 8000 lux, direct sunlight, even some 300 000 Lux In addition, the person is also exposed to artificial light at night stimuli. The so-called "internal clock ", the day of a new " adjustment " needs has, thereby facing problems. The effects may include: sleeping and eating disorders, lack of energy to depression. In very far from the equator regions (such as Norway), where in winter the light output per day can even go to zero, is now the light therapy accepted as effective against seasonal affective disorder ( so-called " light showers " as bright lights, the front of special headgear are attached ). These depressions are the causes but primarily on a lack of daylight and less at the disturbed daily routine.

When changing to different time zones, the own Circadianrhythmus the time zone adjusts. This adjustment can be felt by the so-called jet lag fatigue and weakness. A partial adjustment also takes place in shift work. Also for shift workers exists the phenomenon of adaptation delay. That's why you work in modern alternating shift schedules with so-called " interspersed night shifts ", ie short night shift blocks, ideally consisting of only one or two, but not exceeding three nights.

Plant

Adjustments to the phase activity can also be observed in plants. The important plants for sunrise and thus the beginning of photosynthesis of plants prepared by activation of the photosynthetic apparatus before sunrise. Many plants open and close their flowers at certain times of the day (see the famous flower Linnaeus ). Other plants, their flowers have opened several days in succession, produce perfumes and nectar only at certain times. Pollinating insects like bees make their visits a fact.

More rhythms

• Rhythms with period lengths shorter than 24 hours are called ultradian Rhythms, with longer infra diane Rhythms.