Definition of the circadian rhythm

The circadian rhythm is a natural, internal process that repeats itself and is adapted to the 24-hour day. The circadian rhythm controls the sleep-wake cycle and influences, among other things, body temperature, blood pressure, hormone secretion and the function of the immune system.

The centre of this internal biological clock is the so-called suprachiasmatic nucleus (SCN), a small region in the hypothalamus of the brain. This nucleus receives signals from the eye, in particular about the state of light in the environment. It then uses the light conditions to control the production of the hormones cortisol and melatonin, which significantly influence the sleep-wake rhythm in the circadian rhythm [1,2].

In hormonal terms, circadian rhythms are therefore controlled by the counterparts cortisol and melatonin. The hormones cortisol and melatonin have opposing but complementary effects. This means that they act in opposite directions but complement each other to control the circadian rhythm. In combination, they ensure that the body is optimally prepared for the alternation between wakefulness and sleep.

Cortisol: The wake-up hormone

Cortisol is a hormone that influences a variety of processes in the body. In connection with the circadian rhythm, it is also known as the wake-up hormone. This is because the concentration of cortisol in the blood is highest in the morning [3,4].

Cortisol is produced in the adrenal glands. It plays a key role in preparing the body for the day. Cortisol levels rise sharply in the early morning.

The increase in cortisol levels causes blood pressure and blood sugar levels to rise. Cortisol also stimulates the metabolism. By controlling the energy balance, cortisol promotes physical performance and mental alertness.

During the course of the day, cortisol levels fall continuously until they reach their lowest level in the evening. This drop prepares the body for rest and sleep.

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Melatonin: The sleep hormone

In contrast to cortisol, melatonin is the ‘sleep hormone’ that is produced in the pineal gland of the brain. The production of melatonin is inhibited by light. If these light conditions decrease, melatonin production increases. Light here includes natural daylight and blue light emitted by electronic devices, for example.

Melatonin helps to lower blood pressure, slow down the heart rate and slow down the metabolism. The body is prepared for sleep [4].

Melatonin levels remain high during the night and drop again when daylight ushers in the morning. This inhibits melatonin production and helps the body to prepare for waking up.

The interaction between cortisol and melatonin

The coordination between cortisol and melatonin forms the core of the circadian rhythm. These hormones follow an almost opposite pattern: when melatonin rises in the evening, cortisol falls to prepare the body for sleep. In the morning, cortisol levels increase while melatonin decreases, causing the body to enter the waking state [4].

This finely tuned interplay ensures that the body is active and awake during the day, while at night it gets the rest and recovery it needs. In a healthy circadian rhythm, both hormones work together harmoniously to ensure a stable alternation of sleep and wakefulness.

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Biological processes controlled by the circadian rhythm

The circadian rhythm has far-reaching effects on many bodily functions. Some of the most important biological processes influenced by this rhythm are the sleep-wake rhythm, hormone production, body temperature and metabolic processes.

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Sleep-wake cycle

The best-known process controlled by the circadian rhythm is the sleep-wake cycle.

In the control of the sleep-wake cycle, light is the strongest external factor influencing this cycle. When it gets dark, the SCN signals the pineal gland to release the hormone melatonin, which prepares the body for sleep. Conversely, daylight inhibits melatonin production and promotes wakefulness and activity [1-4].

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Hormone production

In addition to melatonin, the release of other hormones is also controlled by the circadian rhythm. Another important hormone involved in this process is cortisol, also known as the ‘stress hormone’. Cortisol levels typically rise in the early morning, which helps to prepare the body for waking up. Cortisol levels fall throughout the day, while melatonin rises in the evening [1-4]

The feeling of hunger is also controlled by the circadian rhythm. During the day, a feeling of hunger often occurs every four to five hours, whereas this does not occur at night. This process is controlled by the hormones leptin and ghrelin.

The satiety hormone leptin is primarily released by fat cells and reaches its highest concentration in the blood between midnight and 6:00 am. The hunger hormone ghrelin, on the other hand, is primarily produced by the gastric mucosa and pancreas. Its concentration is highest before breakfast [5-7].

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Metabolism

The circadian rhythm also influences the metabolism. The body reacts better to food at certain times of the day, as the digestive enzymes and metabolic processes work in a synchronised cycle. For example, glucose metabolism is more efficient during the day than at night [8,9]. The circadian rhythm also influences the way the body stores and utilises energy, which can have an impact on weight and health [10].

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Body temperature

Body temperature also fluctuates throughout the day. It is low in the morning and peaks in the late afternoon. These fluctuations help to regulate sleep, as a falling body temperature promotes sleep, while a rising body temperature promotes wakefulness and activity [11,12].

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Immune system

Studies show that the immune system is also influenced by the circadian rhythm. Scientific studies indicate that certain immune cells and processes are more active at different times of the day [12,13].

Vaccinations, for example, generate stronger immune responses in the morning than at midday or in the evening, which indicates that the immune system is more active in the morning hours [13].

The intestinal immune system also follows the circadian rhythm: When food is ingested, the intestine is geared towards absorbing as many nutrients as possible. In order to prevent certain substances from crossing the intestinal epithelial barrier, the intestinal immune system increases its antimicrobial functions and stabilises the barrier function [13].

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Disorders of the circadian rhythm

The circadian rhythm is closely linked to our daily activities and general well-being. Disruptions to the sleep-wake rhythm can therefore have serious health consequences. There are various triggers and forms of circadian rhythm disorders, which can be caused by internal or external factors.

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Triggers

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Shift work and jet lag

One of the most common causes of circadian rhythm disorders is shift work. People who work nights and have to sleep during the day force their bodies to work against the natural circadian rhythm. This can lead to sleep disorders, fatigue, and long-term health problems such as cardiovascular disease and metabolic disorders.

Jet lag occurs when a person rapidly crosses several time zones and the circadian rhythm suddenly becomes out of sync with the local day-night cycle. The symptoms of jet lag are sleep disturbances, fatigue, concentration problems and digestive disorders. The body needs a few days to adapt to the new circadian rhythm.

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Chronic stress

In stressful situations, cortisol is released to enable the body to perform at its best. If stress becomes chronic, the cortisol level in the blood can be permanently elevated. This is not only associated with health risks, but can also affect the circadian rhythm. This is because cortisol can suppress melatonin levels, making it difficult to fall asleep. This in turn can lead to insomnia and a shifted sleep rhythm.

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Forms

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Delayed Sleep Phase Syndrome (DSPS)

Another common circadian rhythm disorder is delayed sleep phase syndrome (DSPS). People with DSPS have difficulty falling asleep at normal times. They often wake up later than would be necessary for social or work commitments.

This is particularly common in adolescents and young adults whose internal clocks are shifted backwards.

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Advanced Sleep Phase Syndrome (ASPS)

In contrast to DSPS, early sleep phase syndrome (ASPS) is a disorder in which those affected become tired very early in the evening and wake up very early in the morning.

This disorder is more common in older adults. It can lead to social isolation if ASPS is accompanied by difficulties in adapting to the rhythms of others.

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Irregular Sleep-Wake Rhythm

A less common disorder is Irregular Sleep-Wake Rhythm, where there is no clear cycle of sleep and wakefulness. People with this disorder have multiple sleep periods within a 24-hour day, leading to chronic fatigue and concentration problems.

This disorder is more common in people with neurological disorders such as Alzheimer's disease.

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Influencing the circadian rhythm

There are several ways to influence or stabilise the circadian rhythm.

One of the most effective methods is light therapy, which uses bright light to ‘train’ the SCN and adjust the body to the desired circadian rhythm. This therapy is often used for shift workers, people with jet lag or people with DSPS.

Another way to influence the circadian rhythm is to take melatonin, a hormone that naturally promotes sleep. Melatonin supplements can help regulate the sleep-wake cycle, especially for people who have difficulty falling asleep at night.