Listen To Your Body: How Syncing With The Internal Clock Boosts Health

Sleep, nutrition... All of our body's biological functions are linked to one or more cycles. What are they? And how can we synchronize our lives with the most important one: the circadian rhythm? Our lives, and those of all living beings, are punctuated by various biological rhythms, fundamental to their proper functioning. What are they? 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:

• Ultradian rhythms with a period of less than 24 hours. These are, for example, REM sleep cycles or respiratory or cardiac rhythms.
• Infradian rhythms, which have a period of more than 24 hours, like the menstrual cycle.
• Circadian rhythms. True biological clocks: they run for a duration equivalent (or almost) to 24 hours (circadian coming from the Latin circa , around, and dies , day). Among the best known are the sleep/wake systems or hormone regulation.

This last rhythm, set to 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 encephalon) for our species. It is composed of the 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 by external regulatory agents, such as temperature or food intake... but especially light. Our retina detects 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 - that is, what time of day it is. It is such a powerful regulator that people with total blindness (who therefore do not detect light) have disruptions in their circadian rhythm, associated with significant sleep disorders .

The hormones that regulate our day

In human physiology, a day is considered to be divided into two phases  : activity (from 8-9am to 8-9pm, which corresponds to our workday, school, etc.), and rest (from 8-9pm to 8-9am). They depend on the production of melatonin, known as the "sleep hormone . "

Melatonin secretion is synchronized with the day-night cycle: it begins when light decreases in intensity, around 9-10 p.m. in summer, and reaches its peak 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 increased energy demand necessary for proper functioning during the active phase. Its production is directly linked to the disappearance of melatonin—the mere presence of which inhibits the secretion of cortisol. This allows cortisol production to be synchronized with daylight.

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

• Ghrelin, which stimulates appetite. It is secreted during three peaks, around 8 a.m., 1 p.m. and 6 p.m.
• Leptin, a ghrelin antagonist. Secreted between 4 p.m. and 2 a.m., peaking at 7 p.m., it helps stop eating by inducing satiety and reducing the urge to eat.
• Adiponectin, involved in the regulation of carbohydrate and lipid metabolism. It is produced throughout the day, starting at 10 a.m. After a peak around 11 a.m.-12 p.m., it gradually decreases until nightfall. This hormone promotes the use of energy substrates (sugars and lipids, etc.) to generate the energy needed to support our active phase, 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 decrease. It prepares us for the coming night.

These cyclical productions of light-related hormones are essential for the body's functioning. Other environmental factors can also influence circadian rhythms, including food consumption, which will vary hormone production.

When should we eat to be in line 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 a.m., 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 shortly after by the peak of the satiety hormone (leptin), which is consistent with a signal to stop eating .

So, it seems more consistent to eat from 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 as reserves.

Another point to consider should be seasonal variations. Indeed, in Europe, the duration of melatonin secretion is longer in winter because the days are shorter, and conversely, shorter in summer because 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 as in tune with our biological clock as possible.

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

Circadian rhythm disruptions: What are the consequences for our health?

The modernization of lifestyle, which refers to the transition from traditional, rural life to urban, modern life, is directly linked to the various industrial revolutions and particularly to the invention of artificial light by Thomas Edison in 1879. The 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 encourages shift work.

Additionally, globalization and the development of new technologies have promoted the relocation of businesses, forcing many employees to synchronize their work schedules with the schedules of the countries they work for. Across all scenarios, nearly 30% of employees reported working outside of daytime hours (i.e., between 9 a.m. and 5 p.m.), and 19% of Europeans work at least two hours between 10 p.m. and 5 a.m.

Working night shifts disrupts circadian rhythms, including altering hormone levels. Several studies have shown that night shift workers produce less melatonin than employees with standard shifts . These disruptions are associated with an increased risk of developing cardiovascular disease .

Artificial light also makes it easier to stay awake later, which leads to other behaviors that disrupt the circadian clock:

• Eating dinner after dark, when you get home late from work ( late-night eating in English). As mentioned above, we should no longer eat after 7 p.m. – a practice associated with poor insulin sensitivity, which increases the risk of developing obesity, type 2 diabetes and cardiovascular disease ( especially when these late meals consist of processed ready meals, enriched with fat and sugar ).
• Skipping breakfast, even though morning is a physiologically favorable time to eat. This habit is associated with an increased risk of developing metabolic diseases .
• Changing our sleep schedules. Most people accumulate sleep deprivation during work/school days and compensate by sleeping longer on their days off. This mismatch leads to a circadian clock disorder known as "social jetlag," where the sleep rhythm controlled by the circadian clock no longer matches the actual hours of sleep . This social jetlag, however, again increases the risk of developing obesity and cardiovascular disease .

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

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

As we've seen, circadian rhythms are essential for the proper functioning of our body's metabolism. While modern life, work schedules, and social interactions can sometimes be difficult to align with our biological clock, it's important to remember how it works and try, as much as possible, to live in sync with it.

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

• Try to eat our meals between 8am and 8pm, and limit evening meals or snacks as much as possible.
• Limit our use of screens in the evening and encourage falling asleep by reading on paper, to limit exposure to the blue light from screens . Using a smartphone before falling asleep actually reduces the production of melatonin, while reading a paper book allows for normal secretion of the hormone .
• Try to maintain a regular sleep schedule during the week to limit the risk of "social jet lag." Ideally, you should aim for between 7 and 9 hours of sleep per night .
• Try to respect the rhythm of the seasons, in order to best match your rhythm of life with the cycles of light.

These simple behaviors appear to improve health markers and could be a solution to combat certain metabolic diseases. In any case, the importance of matching lifestyle habits to circadian rhythms is clear and beneficial.

This article by Anouk Charlot, Doctoral student, University of Strasbourg, and Joffrey Zoll, MCU-PH in physiology, Faculty of Medicine, University of Strasbourg in The Conversation.