Circadian rhythmicity is the ability to orient to time in relative independence from external influencing factors. This ability is crucial for bodily functions such as hormone secretion or blood pressure. Abrupt time zone changes throw the clock out of balance and manifest in jet-lag.
What is circadian rhythmicity?
Circadian rhythmicity is the ability to orient to time in relative independence from external factors. Like most other organisms, humans possess an internal clock that allows them to orient themselves to time without looking at an actual clock. The circadian rhythm is also called circadian clock and corresponds to this inner clock. It gives humans the ability to form a picture of time independent of external factors. The circadian clock primarily controls periodically recurring activities, such as sleeping, reproduction or food intake with a certain regularity. These life- and species-sustaining actions thus take place in a comparatively constant rhythm, relatively independent of external factors and actual time awareness. The inner clock adapts to the changing day lengths after a change of seasons by resynchronization. Since the inner clock has to be resynchronized too quickly when traveling to other time zones, there is a lack of agreement at the beginning. This lack of agreement between the internal and actual clock times is also known as jet-lag in the context of long-distance travel.
Function and task
Many vital body functions require periodic coordination. For example, human body temperature must be coordinated in this way. The same is true for blood pressure, heart rate, and urine production. Hormone secretion is also dependent on periodic coordination. Not only sex hormones have to be coordinated periodically. Many absolutely vital bodily functions are also controlled hormonally, and since the hormone balance is a closely interacting system, a miscoordination of a single hormone upsets the entire body and can even have life-threatening consequences. Since the aforementioned bodily functions are not subject to conscious control, they must be independent of the actual conscious knowledge of the time. Therefore, the circadian rhythm is responsible for their control. The human internal clock receives its information from specialized photoreceptors in the granular layer of the retina. The responsible sensory cells are also called photosensitive ganglion cells and are equipped with the photopigment melanopsin. They are located between the ganglion layer and the amacrine cell layer of the retina, where they are connected to the tractus retinohypothalamicus, which projects the collected information from the cells up to the nucleus suprachiasmaticus in the hypothalamus. The nucleus suprachiasmaticus is thus considered the control center for the internal clock. Here, the periodically changing body functions are coordinated in time. At the molecular level, several genes are involved in circadian rhythms, genetically coding for the internal clock, so to speak. In addition to cryptochromes, the CLOCK gene is considered one of the most important genes in this context. The BMAL 1 gene, the PER 1 to 3 genes and vasopressin or prepressophysin are now also known to be important molecular components of the internal clock. In complex interaction, they control both transcription and translation of self-regulatory loops in feedback that occur over a relatively precise 24-hour period. The PER 2 and BMAL 1 genes are light- and temperature-dependent and are transcribed at the onset of the day, for example. They then bind as a dimer to the regulatory sequence of DNA, initiating transcription of the other genes.
Diseases and disorders
Some sleep disorders are related to functional circadian clock complaints. The group of these sleep disorders is often referred to as circadian sleep-wake rhythm disorders. The circadian rhythm is supposed to provide humans with an ideal amount of sleep and thus rest during dark phases. In the light phases, a high level of performance is thus achieved. External stimuli adjust the circadian clock to the 24-hour cycle. Sudden deviations from the usual light-dark change confuse the organism because they occur in an unexpected period of time.Because long-distance flights and time zone changes in particular are accompanied by unexpected light-dark changes for the organism, those affected by circadian sleep-wake rhythm disorders are often regular long-distance travelers. Blind people also often suffer from the disorders because they lack the external factors to synchronize them. The same applies to shift workers, in whom the sleep disorder manifests itself primarily as sleep or fatigue at the “wrong time.” In shift workers, the rhythm of the environment does not correspond to the rhythm of light-dark changes, which leads to problems in synchronizing the internal clock. Chronic circadian sleep disturbances often develop into depression or other mental illnesses. A disturbed internal clock can also be causally related to a mutation of the circadian genes. Such mutations result in longer or shorter periods of activity for the individual, which may deviate from the usual 24-hour rhythm to a greater or lesser extent. Diseases related to the circadian clock have not yet been adequately studied, as even the associated genes are a rather recent discovery. The relationship of circadian rhythms to the aforementioned sleep disorders also requires further research. Studies that deal with the circadian problem ostensibly have been few and far between.
