Better respect your internal clock for better health

Our life, that of all living beings, is marked by various biological rhythms, fundamental for their correct functioning. What is it about? Literally, they correspond to the “periodic or cyclic variation of a specific function of a living being”.

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

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

  • The Infradian rhythmswho have a period longer than 24 hours, such as menstruation.

  • The circadian rhythms. True biological clocks, they work for a period equivalent (or almost) to 24 hours (circadian from Latin approximatelyaround and dies, day). Among the best known are the sleep / wake or hormonal regulation systems.

This latter pace, based on the day, is particularly important. It is directed by an internal “clock”, the operator of which is nested in the brain, and more precisely in the hypothalamus (located below our brain) for our species. It consists of two suprachiasmatic nuclei (located under the optic chiasm), rich in neurons, whose electrical activity fluctuates over 24 hours, controlled by the cyclical activation of particular genes called “clock genes” or “circadian genes” .

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

The hormones that mark our day

In human physiology, we consider that a day is divided into two phases: activity (from 8:00 to 9:00 and from 20:00 to 21:00, and which corresponds to our day of work, school, etc.), and rest (from 20 -21: 00-8: 00-9: 00). 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 21-22 in summer, and reaches its peak of secretion in the middle of the night, between 3 and 4 in the morning; then it decreases until 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 increased demand for energy, which is necessary for proper functioning during the activity phase. Its production is directly linked to the disappearance of melatonin, whose mere presence 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 combination of melatonin-cortisol:

  • Grelin, which stimulates the appetite. It is secreted during three peaks, around 8:00, 13:00 and 18:00.

  • Leptin, ghrelin antagonist. Secreted between 4pm and 2am with a peak at 7pm, 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 all day, from 10:00. After a peak between 11:00 and 12:00, it gradually decreases until sunset. This hormone favors the use of energy substrates (sugars and lipids, etc.) in order to generate the energy necessary to support our activity phase, rather than their accumulation. It is also known to improve insulin sensitivity and prevent fat accumulation.

  • Insulin, which promotes the accumulation of energy substrates. Its production increases during the afternoon, between 2pm and 6pm, when adiponectin begins to drop. It prepares us for the night to come.

Daylight is the largest regulator of the circadian clock, allowing us to secrete our hormones at the right time of day.
A. Charlot, from Beneficial effects of time-limited nutrition on metabolic diseases, Author provided

These cyclical productions of light-bound hormones are essential for the functioning of the body. Other environmental factors can also affect circadian rhythms, particularly food consumption, which will vary hormone production.

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

Knowing your metabolism well offers useful clues to know about your health, as we highlighted in a recent study. If we refer to hormone fluctuations throughout the day, we can assume that we should start the day with breakfast around 8 am, after the cortisol spike, when our activity phase begins. And we should no longer eat after the insulin spike, in the early evening, as this hormone promotes the accumulation in the form of adipose tissue.

Also, the insulin spike is followed shortly after by the satiety hormone (leptin) spike, which suggests a signal to stop eating.

Therefore, it seems more consistent to eat in the morning until late afternoon, when we produce the hormones involved in the use of energy substrates, rather than after 19:00 when we are more likely to store them in the form of reserves.

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

Numerous scientific studies have shown that living out of step with circadian rhythms, particularly by eating late at night, or by changing sleep patterns, increases the risk of developing cardiovascular disease, obesity or type 2 diabetes.

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Circadian rhythm disorders: 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 in particular to the invention of artificial light by Thomas Edison in 1879. T he expansion di artificial lighting has represented a major upheaval in our way of life, as it allows us to work at any time of day or night and favors staggered working hours.

Furthermore, globalization and the development of new technologies have favored the relocation of companies, forcing many employees to synchronize their working hours with the hours of the countries they work for. In all scenarios combined, nearly 30% of employees said they worked outside of daytime hours (i.e. between 9am and 5pm) and 19% of Europeans worked at least 2 hours between 22:00 and 5:00.

Night work causes disruptions to circadian rhythms, particularly by altering hormone levels. Several studies have shown that night workers produce less melatonin than employees with standard hours. However, these disorders are associated with an increased risk of developing cardiovascular disease.

Artificial light keeps you awake even later, which leads to other circadian clock disruption 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 behavior of consumption of sweet products in the evening after a day of work, to relax. Sugar intake activates the 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 try, as much as possible, to live with it at the rhythm.

Working at night or dining late disrupts our internal clock
What helps and what disturbs our biological cycles… Our lifestyles can in fact influence our biological clock: what correct behaviors to adopt?
A. Charlot, from Beneficial effects of time-limited nutrition on metabolic diseases, Author provided

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

These simple behaviors appear to improve health indicators and could be a solution to fighting some metabolic diseases. Either way, the importance of matching lifestyle habits to circadian rhythms is clear and beneficial …

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