BIOLOGICAL CLOCK AND CIRCADIAN RHYTHM – THE RHYTHM THAT SYNCHRONIZES OUR BODY

How does our body “know” when it is time to sleep, wake up, be active, or rest? Our body possesses an internal rhythm that coordinates numerous physiological processes, and understanding this rhythm can help us better organize our day and align our sleep, nutrition, and activities with the needs of our body.

The concept of the biological clock has been known since ancient times and does not apply only to humans; it is a universal phenomenon present throughout the living world – from plants and animals to humans.

One of the earliest recorded observations that is now considered fundamental to the idea of an internal biological rhythm dates back to the 18th century, when the French scientist Jean-Jacques d'Ortous de Mairan observed that the mimosa plant continued its daily leaf movements even when kept in complete darkness. This simple yet revolutionary experiment was the first to suggest the existence of an internal, endogenous rhythm that does not depend directly on external light.

The concept was later developed through the work of numerous scientists, and today we know that almost all living organisms possess biological rhythms that are synchronized with the cycle of day and night.

The Human Biological Clock and Isolation Experiments

When it comes to humans, key studies of circadian rhythms were conducted in the mid- and late 20th century, particularly in Germany at the Max Planck Institute.

These experiments, led by researchers such as Jürgen Aschoff and Rütger Wever, involved healthy adult volunteers – both men and women, most commonly between the ages of 20 and 40.

The participants stayed in specially designed isolation environments (so-called “bunkers”), without access to natural light, clocks, or other time cues. This removed the external signals that normally “set” or synchronize our internal clock.

During these experiments, researchers monitored various physiological parameters, including:

• sleep–wake cycles

• body temperature

• hormone secretion (directly and indirectly through metabolites)

• activity levels and behavior

• perception of time

• blood pressure

These studies demonstrated that the human body possesses an internal rhythm that continues to “run” even in the absence of external signals. Later experiments were also conducted in social environments and showed that individual organs and organ systems have their own daily rhythms according to which they function. The concept was subsequently expanded to include meal timing, the monthly rhythm of hormone activity in women, and other biological processes.

Why Is the Biological Rhythm Important?

The circadian rhythm is not simply a “clock in the brain” – it is deeply connected to our physiology. It influences:

• energy levels throughout the day

• concentration and cognitive functions

• memory and learning

• hormone secretion, particularly melatonin and cortisol

• digestion and metabolism

This is why it is often said that our body does not function at the same level of energy continuously throughout the 24-hour period. Memory, learning, digestion, and metabolism all have their own “waves” – distinct phases of activation and rest.

Why 24 Hours?

Under isolated conditions, the human internal circadian rhythm does not actually last exactly 24 hours. On average, it lasts approximately 24.1 to 24.5 hours, most commonly around 24.2 hours.

This small difference is not an error, but a natural characteristic of the biological system. It results from complex molecular processes involving so-called “clock genes,” as a certain amount of time is required for the synthesis of proteins that regulate the rhythm.

From a biological perspective, this means that our internal clock is slightly “slower” than the astronomical day.

However, we do not notice this difference in everyday life because external factors constantly adjust and synchronize our internal clock. In addition, the rhythm can also be shortened or accelerated under certain conditions.

Light as the Main Regulator

The most important factor that synchronizes our internal clock with the 24-hour day – and therefore influences our feelings of sleepiness and wakefulness – is light.

Light signals from the eyes are transmitted through specialized receptors to the suprachiasmatic nucleus (SCN), a structure in the hypothalamus that functions as the brain’s central biological clock. The SCN then “resets” the rhythm and brings it back into alignment with the external day.

Through this mechanism, our biological clock is synchronized with the environment every day – a process known as circadian entrainment or circadian synchronization.

The Biological Clock and Everyday Life

It is important to emphasize that our internal rhythm is also influenced by lifestyle factors – work obligations, stress, sleep habits, meal timing, and exposure to light.

There is also the concept of the “social clock” – regular working hours, scheduled obligations, fixed lunch breaks at work, and social circumstances. An integral part of our lives is the constant adaptation to socially determined time intervals, which are not always aligned with our biological rhythm. This can lead to so-called desynchronization, resulting in sleepiness or reduced focus at times when we need to be active and functional.

As a result, our rhythm can shift slightly in relation to the “ideal” pattern. In practice, this is reflected in different chronotypes – morning and evening types of people. Our rhythms can also adapt to social circumstances, meaning that certain time intervals may shift by 2–4 hours.

For example, our “clearest window of attention,” when we have the best focus for simple activities such as data entry, may naturally occur between 8 and 10 a.m. However, adaptation to the social clock can shift this window by 2–4 hours, allowing us to remain functional in data-entry tasks until approximately 2 p.m.

On days when we have more freedom, we may notice that we naturally return to our own rhythm – sleeping, waking, and eating in accordance with the internal signals of our body.

Understanding this rhythm does not mean following strict rules. Rather, it means listening more carefully to our own body and recognizing that our organism functions in cycles, not as a “straight line.”

Concluding Thought

The biological clock is not merely a scientific concept; it is a fundamental principle underlying the organization of life. Understanding it means understanding our capacities and potential – as a rhythmic system that can help us better align our obligations and activities with light, time, and the functioning of our internal organs, allowing us to function as effectively as possible while reducing the level of everyday stress.

TO BE CONTINUED…

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