Whether plant, animal or microbial, the living is, by definition, in a perpetual state of becoming; it transforms, adapts, evolves and, above all, is subject to the passage of time. For the living, to extract oneself from time is to extract oneself from life itself. This lived time is that of biological rhythms; organic pulsations whose complexity is barely touched by consciousness and which, nevertheless, give us an intuitive perception of time. But how is the living influenced by these rhythms?
André Klarsfeld, professor at ESCPI and author of The clocks of life (Odile Jacob), was the guest on October 20th of the cycle " Questions of life "which is proposed by the Université Populaire de la Mairie de Paris du 2e as part of the Living Festival. His lecture gave us elements of answers to questions about time, about the mysteries of our internal clock and made us discover the universe of chronobiology, the science of the temporality of biological functions through their alterations and regulations.
Indeed, many of these processes are periodic. Some are seasonal; this is the case of hibernation or connected breeding in the spring. But what we are interested in here are those that follow a circadian rhythm, lasting more or less 24 hours, such as body temperature, hormone secretion and of course the waking-sleeping phases.
A clock in the head
The living has the ability to measure time. This is particularly useful for anticipating the physiological needs of the organism. The drosophila (a biological model particularly prized by chronobiologists) sees its locomotor activity increase before daybreak and nightfall, anticipating dawn and dusk, whereas an arrhythmic mutant fly will only activate suddenly when the light is switched on and off. It is the same with the level of cortisol, the stress hormone, which increases during sleep in order to be at its maximum at the time of awakening.
This "metronome" capability is self-contained. Circadian rhythms, although sensitive to the environment, are not dictated by it; they keep a remarkably constant tempo. Organisms therefore experience an "internal" time that can be described as "subjective", independent of real time (that of day-night cycles or seasons).
The speleologist Michel Siffre during his timeless experience, trapped for two months at the bottom of the Scarasson chasm, without temporal reference points, from July 17th 1962.
The conductor of this imperturbable meter is the internal clock. Thus the tempo is endogenous, and as a result, it can be characteristic of each species; in Man, for example, the duration of the "subjective" day is 25 hours. This could be observed in 1962, when Michel Siffre, a French speleologist, stayed in a cave for several weeks without any temporal marker. Because his perception of time no longer corresponded to solar time, he thought, when he came out on September 17, that it was August 20.
This biological clock was discovered during the birth of computers. We can think that the awareness of the need to organize machines according to a major reference point, time, prompted the search for the temporal supports of the living.
External signals that synchronize
Internal clocks are not dictatorial! They are constantly resetting themselves thanks to environmental signals such as temperature variations and, above all, luminosity. Such variations are naturally found with the day-night alternation, allowing the internal clock to be reset.
The clock is located in the suprachiasmatic nuclei (NSC) in the hypothalamus. In humans, no less than 20,000 neurons generate the expression of the genes of the circadian clock. As their name suggests, NSCs are located above the optical chiasmus, affirming the predominant role of light in the synchronization of the internal clock.
Adaptations to the environment are vital as everyone experiences when travelling. If one leaves France to travel to Quebec, the internal clock will be ahead of time (it will be 11 p.m. in one's head versus 5 p.m. local time). A good way to recover from jet-lag is therefore to submit to the late afternoon light of Quebec, allowing our clock to reset.
Threats from blue light screens
But in our modern world, prolonged exposure to light, especially during the night, can be harmful to our health (risk of sleep disorders, but also cardiovascular disorders...). Our clocks are particularly sensitive to light emanating from the blue spectrum. In other words, our screens keep us awake. In order to overcome this problem, André Klarsfeld informs us that there is an application f.lux which, at nightfall, lowers the blue light levels on our screens.
The increasingly frequent and intimate relationship with screens is not our only source of desynchronization; our lifestyles impose on us a tempo that is often out of sync with our biological clocks. Intensive rhythm, night work, ... Three adults out of 2000 suffer from sleep phase delay syndrome; their biological rhythm is normal but simply out of sync. For example, they prefer to go to bed at 2 a.m. to get up at 10 a.m. Unthinkable in today's working world. For the Academy of Medicine, there is a real public health problem with people forced to work at night since they are much more prone to certain cancers and cardiovascular disorders than the general population.
This kind of discrepancy is all the more pronounced for teenagers, of whom we now know that if they are "late risers", it is because their physiology holds them, and does not allow them to be a morning person. This is why André Klarsfeld had advised the French National Education Ministry, as part of a reflection on school rhythms, to delay the start of classes for middle and high school students.
By exploring our biological relationship to time, chronobiology offers us new opportunities to understand our way of life. It is thus possible, for example, to optimize cancer treatments; certain molecules are more effective at certain times of the day. Knowing our rhythms is not only a way of respecting them, it is also a way of respecting ourselves better.