Biological clocks: How does our body know that time is passing?

2023-09-18 07:38:00, Blog CNA

In April this year, Spanish athlete Beatriz Flamini finally saw the light of the Sun after a 500-day stay in a cave. An absolute record, at least for modern times. She told the media that she lost all sense of time on the 65th day.

But can it really be certain that it was the 65th day?

For comparison, in 1962 the Frenchman Michel Sifre came out of the Skarason cave in Italy after spending 33 days there. In fact, he spent 58 days locked in the cave. But how can isolated human beings keep track of the passage of time, even when cut off from their surroundings?

This is because biological rhythms are at the heart of life, regulating it from the molecular level down to all major parts of the body. This includes not only our sleep/wake cycles, but also our body temperature, hormones, metabolism and cardiovascular system.

And these rhythms have many consequences, even in terms of health. In fact, a number of diseases that are episodic, such as asthma, show more severe symptoms at night, while severe cardiovascular events are more frequent in the morning. Another example is shift work, which disconnects people from their environment.

It can be associated with an increased risk of cancer in workers, causing the WHO itself to label it as a possible cause of carcinogenic diseases. But circadian rhythms also affect how we interact with other species.

For example, African Trypanosomiasis, also called Sleeping Sickness, is a disorder of our daily rhythm caused by the parasite Trypanosoma brucei, whose metabolism has a mechanism similar to our immunity.

A biological clock is an internal mechanism of organisms that, in the absence of an environmental signal, operates according to its frequency. For example, the regular alternation of day and night has favored the evolution of the circadian clock (circa, meaning "approximately" while diem "day" in Latin).

The circadian clock mechanism was first discovered in the fruit fly, also known as Drosophila, in the 1970s. It is based on response phases through the translation of the actions of several genes. Gene A promotes the expression of gene B, which in turn inhibits the expression of gene A, creating an oscillation.

During the day, light induces the reduction of specific factors via a photoreceptor called cryptochrome. However, the precise regulation of the clock is based on a complex molecular and neuronal network that ensures its timing and accuracy.

There is no single overarching clock that would organize all of life, as the genes responsible for the biological clock vary from species to species. But the principle remains the same: genes whose expression fluctuates. Biological rhythms have been observed in all organisms studied to date, which include cyanobacteria (a type of bacteria that obtains energy through photosynthesis), fungi, plants and animals, and of course humans.

Biological clocks: How does our body know that time is passing?

In addition, some auxiliary timing mechanisms synchronize the organism with its environment: light (the most studied to date), temperature and food. Environmental cues in general, and light in particular, help resynchronize the individual.

In humans, light is not directly perceived by the molecular clock, but is captured in the retina and then transmitted via the retino-hypothalamic pathway to a central clock, where it modulates protein synthesis. The system is not infinitely scalable, however: the human body takes roughly a day to adjust to a 1-hour time difference.

Biological clocks: How does our body know that time is passing?

But the circadian clock is not the only clock mechanism that exists in nature. Many biological processes are seasonal, such as the migration of many birds and insects, the reproduction and hibernation of many animal species, and the flowering of plants. This seasonality is generally dictated by several factors, including what is known as a circadian clock in the case of many species. The mechanism of this watch has not yet been determined./ Adapted from CNA