Circadian rhythms: what are they and in what biological functions do they intervene

Human beings are animals of habit. Our lives are structured in patterns that repeat themselves from time to time, and the same is true of our bodily functions. There are certain biological oscillations and rhythms that repeat approximately every 24 hours: are the so-called circadian rhythms, 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 we expose one of the best known examples: that of the sleep-wake cycle. In addition, we tell you what are the main disorders related to these biological rhythms.

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What are circadian rhythms?

Our daily life is sustained by a multitude of routines and patterns that occur with a certain temporal cadence. Normally, we go to bed at night and wake up the next day, 7 or 8 hours later. Our eating habits too are scheduled following a specific daily routine: breakfast, 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 greater than 24 hours (p. ex. the menstrual cycle); ultradian rhythms, those that occur in cycles of less than 24 hours (p. ex. heart rate); and circadian rhythms, which repeat approximately every 24 hours.

Circadian rhythms are internal biological processes of our body that are repeated with a temporal cadence of about 24 hours, as we have commented. These periodic variations or biological rhythms regulate our daily metabolic, hormonal and behavioral activity. Bodily functions as important for survival as regulating body temperature or the sleep-wake cycle they work based on these circadian rhythms.

What characterizes these types of 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, since they have been found in all types of living organisms (from unicellular beings to mammals).

Circadian rhythms have a great adaptive value, since they fulfill the function of "internal clock" through which our body models and builds a representation of external time, with which it is able to establish a coherent model and a concordance between environmental events and the organization of their own biological functions to be able to react to external conditions more or less predictable.

The internal biological clock

In humans, circadian rhythms are generated by 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, receive information of light intensity through photoreceptor cells and ganglion cells of the retinas.

Melanopsin is found in these ganglion cells, a protein involved in the regulation of circadian rhythms and the pupillary reflex, among other functions. This mechanism is found in different "internal clocks" spread over 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 period of time.

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

Ultimately, internal biological clocks are responsible for producing and regulating circadian rhythms. Yes OK the main signal that influences these rhythms is daylight (which can turn genes that control biological clocks on or off), any change in these light and dark cycles can disturb (speeding up or slowing down) the behavior of the clocks, with the consequent deterioration in the functioning of the rhythms circadians.

Circadian rhythms and sleep

Circadian rhythms help us 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. The synchronization of circadian rhythms is based on rhythmic changes in the expression of some genes that control internal clocks.

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

The presence of daylight or noise pollution at night can alter the production of melatonin and, therefore, disrupt circadian rhythms. Likewise, the fact that there are light sources during the sleep process or when entering it may mean that secrete the hormones responsible for initiating the activation process prematurely, causing alterations in the cycle of sleep-wake.

Here are some examples of circadian sleep rhythm disorders.

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Circadian sleep rhythm disorders

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 length of time they need to stay asleep or be awake. The most common are the following:

1. Delayed sleep phase syndrome

People with this syndrome have difficulty falling asleep at a socially acceptable time, and they tend to go to bed late (p. ex. at 2 in the morning). The structure and duration of sleep are normal, but this delay to bedtime causes them generates problems at work, school and social level (being late for work meetings, school, etc.). What's more, people with this syndrome find it difficult to get up and have excessive morning sleepiness.

2. Early sleep phase syndrome

People with this circadian rhythm disorder have normal sleep structure and duration, but go to bed much earlier than is socially stipulated (eg. ex. At 6 pm).

This preview of the sleep phase it usually occurs more in the elderly, but also in young boys. As in delayed sleep phase syndrome, this disorder causes the patient severe evening drowsiness and difficulty staying awake in the evening and at the night.

3. Jet lag syndrome

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

Apparently, symptoms may vary depending on the direction of travel with respect to the axis of rotation of the planet. If the trips are towards the west there is a relative advance in the sleep phase; and if they are to the east, there is a delay. However, on average the internal biological clock can change between 1 and 2 hours each day, although there are people who react better than others to “jet lag” (due to a genetic predisposition).

4. Shift work disorder

This circadian rhythm sleep disorder occurs when a person is forced to be awake during their normal sleep-wake cycle. It usually occurs, mainly, in those workers subject to a regime or shift system, both at night and at dawn or rotating, the latter being the ones that cause the most disturbances. Symptoms include: drowsiness, decreased cognitive abilities, and insomnia.

5. Hypernictemeral syndrome

Hypernictemeral syndrome or sleep-wake disorder other than 24 hours is usually caused by blindness, changes in photosensitivity, or environmental or hormonal factors. This syndrome causes the person to change their sleep pattern on a daily basis, usually 1 to 2 hours later each day. The internal biological clock of these patients tends to configure 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, your preferred sleep period changes every day, usually 1 to 2 hours later each day. For unknown reasons, your internal "clock" tends to keep a "day" of 25 hours.

6. Irregular sleep-wake rhythm syndrome

This circadian rhythm sleep disorder occurs for a variety of reasons: for example, when there are changes in light exposure or age-related brain changes (dementias senile). People with this syndrome often doze off intermittently during each 24-hour period.

Bibliographic references:

• Hodelín Tablada, R., Machado Curbelo, C., & Fuentes Pelier, D. (2010). About 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., Cerón, L. F. Z., Amézquita, C. TO. N., & López, J. TO. V. (2013). Circadian rhythm: the master clock. Alterations that compromise the state of sleep and wakefulness in the area of ​​health. Morfolia, 5 (3).