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Our lives, that of all living beings, are punctuated by various biological rhythms, fundamental for their proper functioning. What is it about ? Literally, they correspond to “the periodic or cyclical variation of a specific function of a living being”.

They can be of three types, depending on their duration:

• THE ultradian rhythms, with a period of less than 24 hours. These are, for example, paradoxical sleep cycles or respiratory or cardiac rhythms.

• THE infradian rhythmswhich have a period of more than 24 hours, like the menstrual cycle.

• THE circadian rhythms. True biological clocks, they run for a period equivalent (or nearly so) to 24 hours (circadian from the Latin circaaround, and dies, day). Among the best known are the sleep/wake or hormone regulation systems.

This last rhythm, based on the day, is particularly important. It is directed by an internal “clock”, whose operator is nestled in the brain, and more precisely in the hypothalamus (located under our brain) for our species. It is made up of two suprachiasmatic nuclei (located under the optic chiasm), rich in neurons, whose electrical activity oscillates over 24 hours, controlled by the cyclic activation of particular genes called “clock genes” or “circadian genes”.

Our circadian clock is constantly resynchronized thanks to external regulating agents, such as temperature or food intake… but above all light. Our retina detects the light signals, which are transmitted to the brain and the internal clock, which then synchronizes the metabolic functions of the different tissues according to the information received – ie what time of day it is. It is such a powerful regulator that people with total blindness (who therefore do not detect light) have disturbances in their circadian rhythm, associated with significant sleep disorders.

The hormones that punctuate our day

In human physiology, we consider that a day is divided into two phases: activity (from 8 a.m. to 9 a.m. to 8 p.m. to 9 p.m., and which corresponds to our day of work, school, etc.), and rest (from 20 -9 p.m. to 8 a.m. to 9 a.m.). They depend on the production of melatonin, known as the “sleep hormone”.

The secretion of melatonin is synchronized with the day-night cycle: it begins when the light decreases in intensity, around 9-10 p.m. in summer, and reaches its peak of secretion in the middle of the night, between 3 and 4 a.m. ; it then decreases until the moment the sun rises.

With the return of light, melatonin ceases to be produced and another hormone, cortisol, takes over. This “stress hormone” prepares the body for the increase in energy demand, necessary for proper functioning during the activity phase. Its production is directly linked to the disappearance of melatonin – the mere presence of which inhibits the secretion of cortisol. This synchronizes cortisol production with daylight.

The activity phase is accompanied by the production of other hormones, in addition to this melatonin-cortisol duo:

• Ghrelin, which stimulates the appetite. It is secreted during three peaks, around 8 a.m., 1 p.m. and 6 p.m.

• Leptin, ghrelin antagonist. Secreted between 4 p.m. and 2 a.m. with a peak at 7 p.m., it promotes the cessation of food intake by inducing satiety and reducing the desire to eat.

• Adiponectin, involved in the regulation of carbohydrate and lipid metabolism. It is produced throughout the day, from 10am. After a peak around 11 a.m. to 12 p.m., it gradually decreases until nightfall. This hormone favors the use of energy substrates (sugars and lipids, etc.) in order to generate the energy necessary to support our phase of activity, rather than their storage. It is also known to improve insulin sensitivity and prevent fat accumulation.

• Insulin, which promotes the storage of energy substrates. Its production increases during the afternoon, between 2 p.m. and 6 p.m., when adiponectin begins to fall. It prepares us for the coming night.

These cyclical productions of light-related hormones are essential to the functioning of the body. Other environmental factors can also influence circadian rhythms, in particular food consumption, which will vary the production of hormones.

When to eat to be in tune with our circadian rhythms?

Knowing your metabolism well offers useful clues to know for your health, as we highlighted in a recent study. If we refer to the oscillations of hormones during the day, we can hypothesize that we should start the day with breakfast around 8 am, after the cortisol peak, when our activity phase begins. And we should no longer eat after the insulin peak, in the early evening, since this hormone promotes storage in the form of adipose tissue.

In addition, the insulin peak is followed soon after by the satiety hormone (leptin) peak, which suggests a signal to stop food consumption.

Thus, it seems more consistent to eat in the morning until late afternoon, when we produce hormones involved in the use of energy substrates, rather than after 7 p.m. when we are more likely to store them. in the form of reserves.

Another point to consider should be seasonal variations. Indeed, in Europe, the duration of melatonin secretion is longer in winter since the days are shorter, and conversely shorter in summer since the days are longer. In theory, as we are a species whose physiology depends on the seasons, we should also adapt our meals to these variations in our environment, to best be in tune with our biological clock.

Numerous scientific studies have shown that living out of step with circadian rhythms, in particular by eating late at night, or by shifting your sleep rhythm, increases the risk of developing cardiovascular diseases, obesity or type 2 diabetes.

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Circadian rhythm disturbances: what consequences for our health?

The modernization of the way of life, which refers to the transition from traditional and rural life to urban and modern life, is directly linked to the various industrial revolutions and particularly to the invention of artificial light by Thomas Edison in 1879. L he expansion of artificial lighting has been a major upheaval for our way of life, since it allows us to work at any time of the day or night, and promotes staggered working hours.

In addition, globalization and the development of new technologies have promoted the relocation of companies, forcing many employees to synchronize their work schedules with the schedules of the countries for which they work. All scenarios combined, nearly 30% of employees said they worked outside daytime hours (i.e. between 9 a.m. and 5 p.m.), and 19% of Europeans worked at least 2 hours between 10 p.m. and 5 a.m.

Night work causes disruptions in circadian rhythms, in particular by altering hormone levels. Several studies have shown that night workers produce less melatonin than employees with standard schedules. However, these disturbances are associated with an increased risk of developing cardiovascular diseases.

Artificial light also keeps you awake later, which leads to other circadian clock-disrupting behaviors:

Finally, although hormones normally regulate food intake at the right time of day, external factors such as stress or frustration encourage the consumption of food at inappropriate times. For example, it is common to observe behaviors of consumption of sweet products in the evening after a day’s work, to relax. Intake of sugar activates reward circuits and releases endorphins, which provide feelings of pleasure and relaxation.

How to be “on time” with your internal clock?

As we have seen, circadian rhythms are essential for the proper functioning of our body metabolism. If modern life, working hours, our social interactions are sometimes difficult to reconcile with our biological clock, it is important to keep in mind how it works and to try, as far as possible, to live in rhythm with it.

Several good habits can be adopted, as our work has shown:

These simple behaviors seem to improve health markers and could be a solution to combat certain metabolic diseases. Either way, the importance of matching lifestyle habits to circadian rhythms is clear and beneficial…