Circadian rhythms: what are they and what biological functions are involved?

What are circadian rhythms? Let's see how they work and in what disorders they are misaligned.

Human beings are creatures of habit. Our lives are structured in patterns that repeat themselves from time to time, and so are our bodily functions. There are certain biological oscillations and rhythms that are repeated approximately every 24 hours: these are the so-called circadian rhythmsThey are related to processes such as the regulation of body temperature or sleep and wakefulness.

In this article we explain what circadian rhythms are and how they work, and give one of the best known examples: the sleep-wake cycle. In addition, we tell you about the main disorders related to these biological rhythms.

What are circadian rhythms?

Our daily life is based on a multitude of routines and patterns that occur with a certain time cadence. Normally, we go to bed at night and wake up the next day after 7 or 8 hours. Our eating habits are also patterned are also patterned according to a specific daily routinebreakfast, lunch, snack and dinner. All these biological rhythms order and give coherence to our daily life.

Chronobiology, which is the science that studies these biological rhythms, classifies them as follows: infradian rhythms, those that occur with a cadence of more than 24 hours (e.g. the menstrual cycle); ultradian rhythms, those that occur in cycles of less than 24 hours (e.g. the heart rhythm); and circadian rhythms, which are repeated approximately every 24 hours.

Circadian rhythms are internal biological processes of our organism that repeat themselves with a temporal cadence of about 24 hours, as we have already mentioned. These periodic variations or biological rhythms regulate our daily metabolic, hormonal and behavioral activity. Body functions as important for survival as the regulation of body temperature or the sleep-wake cycle These circadian rhythms are the basis for our circadian rhythms.

What characterizes these rhythms is that they are self-sustaining and persistent, even in the absence of external or environmental stimuli. They are genetically determined and are not exclusive properties of human beings, as they have been found in all types of living organisms (from unicellular beings to mammals).

Circadian rhythms have a great adaptive value, as they serve as an "internal clock". by means of which our organism models and constructs a representation of external time, with which it is able to establish a coherent pattern and concordance between environmental events and the organization of its own biological functions in order to be able to react to more or less predictable external conditions.

The internal biological clock

In humans, circadian rhythms are generated thanks to an internal biological clock located in the an internal biological clock located in the hypothalamus, specifically in the suprachiasmatic nuclei.. This group of neurons located in the medial part of the hypothalamic structures receives information on light intensity from photoreceptor cells and retinal ganglion cells.

Melanopsin, a protein involved in the regulation of circadian rhythms and the pupillary reflex, among other functions, is found in these ganglion cells. This mechanism is found in different "internal clocks" distributed in various tissues, called peripheral oscillators. These clocks are capable of structuring a temporal order in different activities of the organism, so that they oscillate with a regular time period.so that they oscillate with a regular period of time.

These oscillations in time are used by the organism as a time reference to regulate the various biological rhythms of bodily functions, such as the regulation of body temperature, Blood pressure, oxygen consumption or the sleep-wake cycle.

In short, internal biological clocks are responsible for producing and regulating circadian rhythms. While the main signal influencing these circadian rhythms is the main signal that influences these rhythms is daylight, daytime (which can activate or deactivate the genes that control biological clocks), any change in these light and dark cycles can disrupt (accelerating or decelerating) the behavior of the clocks, with a consequent deterioration in the functioning of circadian rhythms.

Circadian rhythms and sleep

Circadian rhythms help us to structure sleep patterns, in what we call the wake-sleep cycle. The main biological clocks located in the suprachiasmatic nucleus produce melatonin, a substance that acts as a sleep regulator, among other functions.among other functions. The synchronization of circadian rhythms is performed based on rhythmic changes in the expression of some genes that control the internal clocks.

The effect of melatonin also follows a pattern: during the night there is an increase in melatonin secretion and a general decrease in neurobehavioral functions. This increase in melatonin levels correlates with an increase in drowsiness and also with a decrease in body temperature. In turn, an increase in blood flow to the more distal regions of the skin is induced, with consequent heat loss.

The presence of daylight or noise pollution during the night can alter melatonin production and thus disrupt circadian rhythms. Likewise, the presence of light sources during the sleep process or at the onset of sleep may mean that hormones responsible for initiating the activation process are secreted prematurely, causing disturbances in the sleep-wake cycle.

Below, we will look at some examples of circadian rhythm sleep disorders.

Disorders of the circadian rhythm of sleep.

Circadian sleep rhythm disorders are disturbances that occur in the sleep-wake cycle when there is a mismatch between a person's sleep pattern and the time he or she needs to stay asleep or awake. The most common are the following:

Delayed sleep phase syndrome 2.

People with this syndrome have difficulty falling asleep at a socially acceptable time and often go to bed late (e.g. at 2 a.m.). The structure and duration of sleep are normal, but this late bedtime causes problems at work, school and socially (being late for work meetings, school, etc.). In addition, people suffering from this syndrome find it difficult to get up and have excessive morning sleepiness..

2. Sleep phase advance syndrome

People with this circadian rhythm disorder have a normal sleep structure and sleep duration, but go to bed much earlier than socially stipulated (e.g. at 6 p.m.).

This earlier sleep phase is more common in the elderly, but also in young children.. As in the case of delayed sleep phase syndrome, this disorder causes the patient to be very sleepy in the evening and to have difficulty staying awake in the afternoon and at night.

3. Jet lag syndrome

Jet lag or jet lag syndrome is one of the best known circadian rhythm disorders and occurs when the internal biological clock remains fixed to the sleep-wake cycle of the time zone in which the person has previously been. Symptoms include difficulty falling asleep at a socially acceptable time and daytime sleepiness.

It appears that the symptomatology may vary depending on the direction of travel with respect to the planet's rotation axis.. If travel is westward, there is a relative advance in the sleep phase; and if travel is eastward, there is a delay. All in all, on average the internal biological clock can change between 1 and 2 hours every day, although there are people who react better than others to jet lag (due to a genetic predisposition).

4. Shift work disorder

This circadian sleep rhythm disorder occurs when a person is forced to be awake during his or her usual sleep-wake cycle. It tends to occur mainly in those workers who are subject to a shift regime or system, either night, early morning or rotating, the latter being those who are forced to be awake during their usual sleep-wake cycle.The symptoms include: drowsiness, drowsiness at night, drowsiness during the night, drowsiness in the early morning or rotating shifts, the latter being the most common cause of sleep disorders. Symptoms include: drowsiness, decreased cognitive abilities and insomnia.

5. Hypernictemeral syndrome

Hypernictemeral syndrome or non-24-hour sleep-wake disorder is usually caused by blindness, changes in photosensitivity, environmental or hormonal factors. This syndrome causes the person to change his or her sleep pattern on a daily basis, usually by 1 to 2 hours.The internal biological clock of these patients is usually 1 to 2 hours later each day. The internal biological clock of these patients tends to set the duration of 1 day as 25 hours.

It can occur for many reasons. The most common cause is blindness, but there are others such as changes in photosensitivity, environmental and hormonal factors. Because of this problem, their preferred sleep period changes every day, generally between 1 and 2 hours later each day. For unknown reasons, their internal "clock" tends to maintain a 25-hour "day".

6. Irregular sleep-wake rhythm syndrome

This circadian sleep rhythm disorder occurs for various reasons: for example, when there are changes in light exposure or age-related changes in the brain (senile dementia). People suffering from this syndrome usually doze intermittently during each 24-hour period.

Bibliographic references:

• Hodelín Tablada, R., Machado Curbelo, C., & Fuentes Pelier, D. (2010). On wakefulness and sleep. Rev Neurol, 51(12), 766-7.
• Richter HG, Torres-Farfán C, Rojas-García PP, Campino C, Torrealba F, Serón-Ferré M.The circadian timing system: making sense of day/night gene expression. Biol Res. 2004;37(1):11-28.
• Torres, J. S. S. S., Cerón, L. F. Z., Amézquita, C. A. N., & López, J. A. V. (2013). Circadian rhythm: the master clock. Alterations that compromise sleep-wake state in health care. Morfolia, 5(3).