What Is the Biological Clock and How Does It Work?

Biological clocks dictate our entire existence from the time we wake up until we go to sleep. Do you want to know what they consist of?
What Is the Biological Clock and How Does It Work?
Samuel Antonio Sánchez Amador

Written and verified by el biólogo Samuel Antonio Sánchez Amador.

Last update: 02 July, 2023

Living beings require a certain degree of routine in our lives. We go to bed at socially pre-established hours, eat at specific times of the day, and assign specific periods of time to work and play. Although all this organization seems like the result of chance, in reality, we have an internal biological clock that dictates all our actions from the moment we get up.

The term “biological clock” can lead to some confusion, because sometimes we attribute certain connotations to it that go beyond the capabilities of this system. What’s more, it’s difficult for us to understand to what extent human behavior is determined by a series of cycles and rhythms. Keep reading to learn more about these mechanisms and how our internal clock works.

What is a biological clock?

As indicated by the National Institute of General Medical Sciences, the biological clock is a natural time device of the living body that regulates the cycle of circadian rhythms. In other words, it’s a biological parameter dependent on a series of internal mechanisms that are synchronized with environmental events.

More than a single “biological clock”, the body of all living things consists of several “clocks”, which refer to specific proteins that interact with cells throughout the body. Almost all tissues and organs in the human body have biological clocks, and the genes that encode them are very similar in humans, plants, mice, fungi, and many more organisms.

Although biological clocks represent an abstract function present in almost all tissues, it should be noted that there’s a place where all this activity is centralized. Specifically, we refer to a group of about 20,000 neurons that make up the suprachiasmatic nucleus. We’ll tell you about the peculiarities of this structure in the following lines.

The suprachiasmatic nucleus: The central clock of the human organism

The biological clock and neurons.
The human brain contains multiple “nuclei” responsible for vital functions. This is the case with the suprachiasmatic nucleus, an important structure for regulating circadian cycles.

As studies indicate, the suprachiasmatic nucleus (SCN) is the body’s main biological clock. Through various projections, this structure synchronizes peripheral rhythms and stimulates the pineal gland through a polysynaptic pathway so that it releases melatonin, a hormone that plays an essential role in sleep cycles (among other things).

The NSQ is located in the medial hypothalamus section and is made up of about 20,000 neurons dorsal to the optic chiasm (hence its name). In turn, the suprachiasmatic nucleus can be divided into two sections: Ventrolateral (nucleus) and dorsolateral (cortex).

These sections differ in the expression of their genes and in the way they synchronize with external rhythms. While the nucleus of the NSQ responds to stimuli, the cortex expresses genes constitutively. That is, they’re permanently transcribed, regardless of environmental conditions.

When we speak of the “expression” of genes, we refer to the mechanisms of transcription and translation. Simply put, sometimes proteins encoded by the genome are synthesized after exposure to an environmental stimulus (thus expressing the encoded event), while in other cases, the synthesis of protein material is uninterrupted.

The suprachiasmatic nucleus contains several cell types and produces diverse peptides and neurotransmitters.

The pineal gland

The Cancer.gov website defines the pineal gland as “A tiny organ in the cerebrum that produces melatonin. Also called pineal body and pineal organ”, although their function is always the same. Histologically, it’s described as a small reddish-gray endocrine gland (which releases hormones into the blood) that’s the size of a grain of rice.

Understanding the role of the pineal gland in the central biological clock is very complex, so we’ll summarize it in a couple of basic ideas. We’ll begin by highlighting the fact that the photosensitive cells present in human eyes detect the presence of light during the day and send the relevant signals to the suprachiasmatic nucleus.

The circadian signals that are created travel to the paraventricular nucleus, the spinal cord, the superior cervical ganglion, and finally, the pineal gland. This small structure interprets the information provided by the NSQ and consequently produces (or doesn’t produce) melatonin, which today we know as “the sleep hormone”. Simply put, the less light there is, the more melatonin is synthesized.

The peak of circulating melatonin is reached between 2:00 and 4:00 in the morning. When it’s daytime and we’re active, the amount of this hormone is minimal.

The biological clock and rhythms

Now you know what the central biological clock is and how it regulates our sleep cycle in a superficial way. In any case, it should be noted that this intricate system doesn’t work by itself, as the existence of certain “rhythms” or “patterns” is necessary in order to influence its operation.

These parameters are known as biological rhythms and refer to a series of oscillations in the environmental and internal variables of the organism. The rhythms modulate us as living beings, as all our vital activities always manifest themselves with a regular variation (and not as an immovable continuum). In the following lines, we’ll explore the different types of rhythms that exist.

Cardiac rhythms

Without a doubt, these are the most well-known in this field. The NIH defines these parameters as “oscillations of biological variables at regular intervals of time.” All living beings have circadian rhythms related to the biological clock that repeat cyclically every 24 hours. These start in the body (endogenous) and respond to external stimuli (exogenous).

For a circadian rhythm to be considered as such, it must meet the following characteristics:

  1. It’s endogenous and persists without the presence of time cues: Circadian rhythms repeat every 24 hours even though environmental conditions are constant (for example, darkness throughout the day). Although it’s true that these parameters respond to stimuli external to the animal’s body, they’re capable of persisting regardless of the environment.
  2. It’s susceptible to synchronization (entrainment): Circadian rhythms can be reset if the body is exposed to certain environmental conditions, such as light and heat. A very clear example of this takes place when we travel from one continent to another; although conditions change and routines adapt and are maintained.
  3. It becomes desynchronized in the face of certain disruptive conditions: If a human being is exposed to bright light for 24 hours, they’ll continue to maintain their circadian rhythms as a concept, but these will be disrupted and won’t work well. This means that they exist independently of the environment, but are modulated by it.

Circadian rhythms are regulated by the biological clock (or biological clocks) present in the body. The mechanism that best exemplifies this terminological conglomerate is the suprachiasmatic nucleus-pineal gland-melatonin circuit. This depends on the photoperiod, and the amount of this hormone synthesized varies according to the hours of light to which the body is exposed.

However, it should be noted that circadian rhythms go far beyond the production of melatonin. The secretion of these hormones is also modulated by the following substances:

  • Adrenocorticropic hormone
  • Thyroid stimulating hormone
  • Cortisol
  • Follicle-stimulating hormone
  • Luteinizing hormone

Lunar rhythms

Although circadian rhythms are the most well-known, they’re not the only ones that exist. For example, lunar variations (selenian rhythms) are very important to explain certain behaviors in living beings. Some biological cycles occur in a specific phase of the moon, while others occur in a specific cycle or in the middle of it.

Several studies have explored the effect of lunar cycles on human sleep and behavior, but the results remain controversial. To date, much remains to be studied to achieve the same level of knowledge in this area as in that of circadian rhythms.

Some jellyfish of the genus Alatina synchronize to mate based on the moon phase. This has been scientifically proven.

Other rhythms and the biological clock

A women's biological clock.
We usually associate biological clocks solely with sleep, but there are many more examples. One of the most representative is the menstrual cycle.

Next, we’ll bring you a list of other rhythms that are neither lunar nor circadian. It’s to be expected that these also affect the functioning of the human biological clock, but much remains to be investigated:

  • Infraradian rhythms: These last more than 24 hours. A very clear example in this area is the menstrual cycle (which lasts 1 month).
  • Ultradian rhythms: Last less than 24 hours. Nasal decongestion cycle takes place every 4 hours.
  • Tidal rhythms: Tides rise and fall in 12-hour intervals. Marine animals adjust their period of activity according to this parameter.

In all biological rhythms, the period of greatest activity is known as acrophase. On the other hand, when the internal processes are less active, it’s said that the living being is in the bathiphase. The amplitude records the difference between the two periods along the biological rhythm.

The biological clock and the human mind

The biological clock controls circadian rhythms and, in turn, the latter modulate the body’s physiological responses. The result after exposure to the stimuli seems quite direct (if melatonin is produced and it’s dark, the human falls asleep), but that’s not quite the case. In fact, science is increasingly realizing that the biological clock goes far beyond hormone production.

A very interesting study published in the journal Frontiers in Behavioral Neuroscience explored the role of the biological clock in the anger and aggression circuits of living beings. For this, the following hypotheses were postulated:

  1. There are predictable rhythms to aggressive and angry behaviors in animals.
  2. The disruption of circadian rhythms encourages aggressiveness.
  3. The chronic expression of anger could impede the proper functioning of physiological rhythms. This could promote the appearance of certain conditions, such as cardiovascular disease.

Seasonal variation in the emotional domain has been demonstrated in nonhuman animals according to infradian rhythms. In general, living things become much more territorial and hostile when they’re selecting territories or mating. In other words, their biological clock dictates that they must attack others of their species (or of different species).

In our species, much remains to be investigated, but it’s intuited that certain human beings with specific circadian rhythms (different chronotypes) are more prone to aggression and aggressiveness. More studies are required in this area, but it’s hoped that a strong correlation will soon be found.

Circadian rhythms can modulate the release of certain neurotransmitters related to the human mood.

The biological clock and circadian rhythms: The environment gives us life

In this space, we’ve dealt with quite complex concepts, but they’re all summarized in one premise: The center of the biological clock is the suprachiasmatic nucleus (NSQ), and there are many other “subunits” that compose it in almost all tissues of the body. Circadian rhythms are modulated by this set of biological tissues and are endogenous.

Depending on environmental conditions, circadian rhythms can be out of adjustment or vary, but they’re always going to be there, even if the conditions are always the same. Without a doubt, the best example that comes to mind is traveling from one country to another: Despite the initial jet lag, you’re able to adapt to the environment, right? Now you know that your biological clock is responsible for this and many more things.

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