Circadian Rhythms: What They Are And What Biological Functions They Are Involved In
Human beings are animals of habit. Our lives are structured in patterns that repeat themselves from time to time, and the same goes for our bodily functions. There are certain oscillations and biological rhythms that repeat approximately every 24 hours: They are 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 the main disorders related to these biological rhythms are.
Our daily life is based on a multitude of routines and patterns that happen with a certain temporal cadence. Normally, we go to bed at night and wake up the next day, after 7 or 8 hours. Our eating habits too They are scheduled following a specific daily routine: breakfast, lunch, snack and dinner. All these biological rhythms order and give coherence to our daily lives.
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 (e.g. the menstrual cycle); ultradian rhythms, those that occur in cycles of less than 24 hours (e.g. heart rate); and circadian rhythms, which repeat approximately every 24 hours.
Circadian rhythms are internal biological processes of our body that repeat with a temporal cadence of about 24 hours, as we have mentioned. These periodic variations or biological rhythms regulate our daily metabolic, hormonal and behavioral activity. Body functions as important for survival as regulating body temperature or the sleep-wake cycle They function based on these circadian rhythms.
What characterizes this type of rhythm 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 great adaptive value, since they serve as an “internal clock” through which our organism models and builds a representation of external time, with which it is capable of establishing a coherent model and agreement between environmental events and the organization of its own biological functions 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 hypothalamus, specifically in the suprachiasmatic nuclei This group of neurons located in the medial part of the hypothalamic structures, receive information about light intensity through the photoreceptor cells and retinal ganglion cells.
In these ganglion cells is found melanopsin, a protein involved in the regulation of circadian rhythms and the pupillary reflex, among other functions. 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 period of time.
These oscillations in time are used by the body as a temporal 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. Yes ok The main signal that influences these rhythms is daylight (which can activate or deactivate the genes that control biological clocks), any change in these light and dark cycles can disturb (accelerating or decelerating) the behavior of the clocks, with the consequent deterioration in the functioning of circadian rhythms.
Circadian rhythms and sleep
Circadian rhythms help us structure sleep patterns, in what we call the sleep-wake 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 carried out 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 drowsiness and, also, with the decrease in body temperature. In turn, an increase in blood flow is induced towards the most distal regions of the skin, with the consequent loss of heat.
The presence of daylight or noise pollution at night can alter melatonin production and, therefore, disrupt circadian rhythms. Likewise, the fact that there are light sources during the sleep process or entering it can mean that the hormones responsible for starting the activation process are secreted ahead of time, causing alterations in the sleep-wake cycle.
Next, we will look at some examples of circadian rhythm sleep disorders.
Circadian rhythm sleep disorders
Circadian rhythm sleep disorders are alterations that occur in the sleep-wake cycle when there is a mismatch between a person’s sleep pattern and the time they need to stay asleep or be awake. The most common are the following:
1. Delayed sleep phase syndrome
People who suffer from this syndrome have difficulty falling asleep at a socially acceptable time, and often go to bed late (for example, at 2 in the morning). The structure and duration of sleep are normal, but this delay at bedtime causes problems at work, school and social level (arriving late to work meetings, school, etc.). Besides, People who suffer from 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 (e.g. at 6 pm).
This advance of the sleep phase It usually occurs more in the elderly, but also in young children As occurs in delayed sleep phase syndrome, this disorder causes the patient to experience great evening sleepiness and difficulties staying awake during 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 in the sleep-wake cycle of the time zone in which the person has previously been. Symptoms include: difficulties falling asleep at a socially acceptable time and daytime sleepiness.
Apparently, The symptoms may vary depending on the direction of travel with respect to the planet’s axis of rotation If the trips are towards the west, there is a relative advance in the sleep phase; and if they are towards the east, there is a delay. All in all, 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 stay awake during their usual sleep-wake cycle. It usually occurs mainly in those workers subject to a regime or shift system, both at night and in the early morning or rotating, the latter being the ones that cause the most disruption. Symptoms include: drowsiness, decreased cognitive abilities, and insomnia.
5. Hypernictemeral syndrome
Hypernictemeral syndrome or sleep-wake disorder other than 24 hours usually occurs due to blindness, changes in photosensitivity, or due to environmental or hormonal factors. This syndrome causes the person to change their sleeping pattern daily, generally 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 maintain a 25-hour “day.”
6. Irregular sleep-wake rhythm syndrome
This disorder of the circadian rhythm of sleep occurs for various reasons: for example, when there are changes in light exposure or there are age-related changes in the brain (senile dementia). People who suffer from this syndrome usually doze intermittently during each 24-hour period.
