Human beings are animals 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: 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 present one of the best known examples: the sleep-wake cycle. In addition, we tell you what are 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 happen at 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 too are guided by a specific daily routine : breakfast, lunch, afternoon 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 rate); and circadian rhythms, which are repeated approximately every 24 hours.

Circadian rhythms are internal biological processes of our organism that repeat themselves with a temporary cadence of about 24 hours, as we have commented. 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 function on the basis of 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 the exclusive property of humans, since they have been found in all types of living organisms (from single-celled beings to mammals).

Circadian rhythms have a great adaptive value, since they fulfil the function of “internal clock” by means of which our organism models and builds a representation of external time, with which it is able to establish a coherent model and an agreement between environmental events and the organization of its own biological functions in order to react to more or less predictable external conditions.

The internal biological clock

In human beings, circadian rhythms are generated by an internal biological clock located in the hypothalamus, specifically in the supraquiasmatic nuclei . This group of neurons located in the medial part of the hypothalamic structures receive information on light intensity through the photoreceptor cells and the retinal ganglion cells.

In these ganglion cells we find melanopsin, a protein involved in the regulation of circadian rhythms, and pupil 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 organism as a time reference to regulate the various biological rhythms of body functions, such as: the regulation of body temperature, blood pressure, oxygen consumption or the sleep-wake cycle.

In short, the internal biological clocks are responsible for producing and regulating circadian rhythms. Although the main signal that influences these rhythms is daylight (which can activate or deactivate the genes that control the biological clocks), any change in these cycles of light and darkness can disturb (accelerate or decelerate) the behaviour of the clocks, with the consequent deterioration in the functioning of the circadian rhythms.

Circadian Rhythms and Sleep

Circadian rhythms help us structure our sleep patterns, in what we call the wakeful-sleep cycle. The main biological clocks located in the supraquiasmatic 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 the 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 a decrease in body temperature. In turn, an increase in blood flow to the most distant regions of the skin is induced, 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 when entering it may mean that the hormones responsible for initiating the activation process prematurely are secreted, causing alterations in the sleep-wake cycle.

Below are some examples of circadian rhythm sleep disorders.

Circadian rhythm sleep disorders

Circadian rhythm sleep disorders are disturbances 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 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 often go to bed late (eg, at 2am). The structure and duration of sleep is normal, but this delay in going to bed causes problems at work, school, and in society (arriving late to 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 Advancement Syndrome

People with this circadian rhythm disorder have a normal sleep structure and duration, but go to bed much earlier than socially prescribed (eg, at 6pm).

This advancement of the sleep phase is usually more pronounced in the elderly, but also in young children . As in the case of delayed sleep phase syndrome, this disorder causes the patient a great deal of sleepiness in the evening and difficulty in staying awake in the afternoon and at night.

3. Jet lag syndrome

Jet lag 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 been previously. Symptoms include: difficulty falling asleep at a socially acceptable time and daytime sleepiness.

It seems that the symptomatology can 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. On average, however, the internal biological clock can change between 1 and 2 hours every day, although some people 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 their usual sleep-wake cycle. It tends to occur mainly in those workers subject to a regime or shift system , both at night and in the early morning or on a rotating basis, the latter being the most disturbing. 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 daily , usually 1-2 hours later each day. The internal biological clock of these patients tends to set the duration of 1 day as 25 hours.

It can happen 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 1-2 hours later each day. For unknown reasons, your internal “clock” tends to maintain a 25-hour “day”.

6. Irregular sleep-wake rhythm syndrome

This circadian sleep rhythm disorder occurs for several reasons: for example, when there are changes in exposure to light or age-related changes in the brain (senile dementias). People with this syndrome often doze off and on for 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., Cerón, L. F. Z., Amézquita, C. A. N., & López, J. A. V. (2013). Circadian rhythm: the master clock. Alterations that compromise the state of sleep and wakefulness in the area of health. Morpholia, 5(3).