What do sleep, digestion, alertness and body temperature have in common? The circadian rhythm! We all have an internal clock, a kind of conductor that governs our biological functions and, consequently, our behavior. How does this clock adjust? How important is it to our health? That's what we're going to find out in this article.
What is a circadian rhythm?
Alongside the infradian and ultradian rhythms, the circadian rhythm is one of the three types of biological cycle in a living being. Lasting around 24 hours, the circadian rhythm regulates numerous physiological processes such as sleep, temperature, blood pressure, hormone production and metabolism.
This rhythm is controlled by a central clock, located in the hypothalamus and known as the suprachiasmatic nucleus (SCN), and by peripheral clocks, located in numerous tissues and organs (muscles, liver, pancreas, kidneys, lungs, etc.).
This system, known as the " internal clock ", has been shown to have its own rhythm, between 23:30 and 24:30 depending on the individual. To be in harmony with its environment and the alternation of day and night, the internal clock constantly resynchronizes itself on a 24-hour cycle thanks to external factors, called " zeitgebers ", and in particular natural light.
In concrete terms, the central clock receives light signals from the retina to synchronize with this 24-hour cycle. It then communicates with the peripheral clocks via several channels:
- Neuronal : sending electrical impulses via the autonomic nervous system (the nervous system that regulates the body's involuntary functions) directly to target organs and tissues.
- Hormonal or humoral: regulation of hormone secretion, such as cortisol or melatonin, which act remotely on these peripheral clocks.
While peripheral clocks are influenced by these signals from the NSC, they are also influenced by other zeitgebers (e.g. the presence of nutrients) and communicate back to it.
In a way, the circadian system enables the temporal division of the body'sphysiological functions in order to optimize their efficiency according to the time of day, which is more conducive to physical activity or recuperation. It regulates processes such as digestion or sleep/wake alternation, and influences hormone secretion according to the time of day or night. Melatonin, for example, a hormone produced by the pineal gland that signals darkness and helps regulate the sleep-wake cycle, begins to be produced at the end of the day to help people fall asleep, and peaks between 2 and 4 a.m. to induce deep sleep (necessary for physical recovery, memory consolidation and strengthening of the immune system), before dropping to almost zero in the early hours of the morning to encourage wakefulness ; cortisol, for its part, peaks on awakening to stimulate the energy and alertness required for wakefulness; still other hormones influence hunger depending on the time of day.
According to research, almost all biological functions are subject to this circadian rhythm. Clearly, the body organizes itself to know when to digest, when to rest, when to secrete this or that hormone according to the usual rhythm of our internal clock , which plays the role of conductor. If this conductor deviates too much from his score, for example if we sleep during the day or eat at night, it's cacophony. Admittedly, our bodies are well designed, able to adapt to our changing habits, but when these changes are too frequent, or when these habits are too far removed from our nature as a diurnal species, this capacity for adaptation is exceeded, and this can affect our long-term health and well-being. Synchronizing the internal clock with the environment is therefore essential to maintaining good health.
Circadian rhythm disorders: the vital role of sleep
Certain pathologies, such as obesity, type 2 diabetes, mood disorders, sleep disorders and even certain neurodegenerative diseases, have been associated with dysfunctions in circadian clocks. Sometimes these dysfunctions are due to genetic mutations in the clock genes, but very often they are caused by external factors that can be influenced.
In our modern society, lack of exposure to natural daylight and, on the contrary, exposure to artificial light at night, particularly that generated by screens, alter melatonin production. As a result,falling asleep is delayed and our natural rhythms are thrown out of sync.
A study published in 1999 in the Lancet showed a decrease in insulin sensitivity, alterations in TSH secretion (a hormone that stimulates the thyroid to secrete thyroid hormones) and an increase in nocturnal cortisol after just six nights of sleep reduced to 4h per night. Other studies have shown that sleep restriction leads to an increase in white blood cells, mainly neutrophils. Immune function could therefore also be affected by sleep deprivation. Still other studies have observed that sleep restriction leads to increased levels of ghrelin (a hormone that stimulates appetite),decreased levels of leptin (a hormone that controls the sensation of satiety) and increased appetite, particularly for fatty and sugary foods. It therefore also influences eating habits.
Some people are obviously more sensitive to the negative effects of sleep deprivation, many factors come into play, and more studies are needed to better understand what's at play, but it's reasonable to think that disturbed sleep can have multiple health consequences.
Taking care of your circadian rhythm
Bedtime:
Here are a few tips to help you improve the quality of your sleep:
- Sleep in a dark, quiet, cool room (around 18°C; 20°C for the elderly).
- Go to bed at a regular time, 2 to 3 hours after dinner, to complete digestion.
- Get enough sleep - about 8 hours for most people.
- Take naps if you feel the need, but limit them to 20 minutes and not after 3pm.
- In the event of stress, to avoid ruminations and bring emotions down, practice breathing exercises. For example, ventral breathing: lie in bed with one hand on your stomach and feel it rise as you inhale and fall as you exhale; cardiac coherence: 5 minutes three times a day, with 6-second inhalations and 6-second exhalations. Some people will appreciate the "bedtime stories" or relaxations offered by certain apps (notably Natflow) or by disconnected devices such as Zenspire if you prefer to leave the phone outside the bedroom.
Light:
Exposure to light is crucial to the proper synchronization of circadian rhythms. Here again, there are a few best practices:
- Exposure to natural morning light. Alternatively, the use of a light therapy lamp can help those who are unable to expose themselves to light on a regular basis. In this case, we recommend using a lamp with a power of 10,000 lux and classified as a medical device (brands Beurer, Philips...).
- Reduce exposure to artificial light, especially the blue light generated by screens such as phones, tablets and computers. To limit this exposure, for example for people who work all day on a computer, the use of blue-light filtering glasses may be an option. In any case, it's advisable to avoid these screens in the evening, and to turn them off around 2 hours before bedtime to avoid disrupting sleep. Some people even replace white light (e.g. on their bedside lamp) with red bulbs in the evening, as this color excites the brain less, doesn't affect melatonin production and helps relax the nervous system, promoting recuperative sleep.
Food :
Nutrients act like zeitgebers on the internal clock, so it will be important to get into good habits:
- Eat a light evening meal, don't overload on proteins and avoid fast sugars (sweet products), which tend to irritate.
- Focus on proteins and good fats (e.g. vegetable oils such as rapeseed or oilseeds) at the start of the day to boost energy and alertness.
- Maintain a regular meal schedule, even during shifts, to stabilize the biological clock.
- Stop taking stimulants (tea, coffee, caffeine-rich soft drinks...) from 4pm onwards, or 2pm for the more sensitive. Note that for some people, dark chocolate can also be a stimulant.
Physical activity:
Sport is good for your health, reduces stress and can help compensate for the negative effects of a disrupted circadian rhythm. For a good night's sleep, we recommend :
- Exercise during the day. Ideally in the morning, in natural light.
- Avoid exercising in the evening, or opt for gentle activities that do not increase heart rate or body temperature. Intense activities should be performed before 6 p.m. to avoid disturbing sleep. Activities based on gentle movement and breathing can, on the contrary, improve relaxation and contribute to quality sleep.
Focus on special chronotypes
Circadian rhythms are highly variable between individuals. Some people are said to have a " morning chronotype ", as they tend to get up and go to bed early; others have a " vesper chronotype ", as they get up and go to bed late. Chronotype is determined not only by genetics, but also by environmental factors such as light exposure and mealtimes.
In the case of the vesper chronotype, some general advice will be difficult to apply. If the person suffers from this chronotype because their professional life, for example, requires them to get up early, it is in their best interest to try to resynchronize themselves to the rhythm of their environment. To do this, she can follow the advice given above, particularly that concerning exposure to natural light in the morning, but gradually. She can start by eating a light breakfast, gradually becoming more and more complete. In the same way, she can gradually reduce the consistency of her dinner (sometimes we're not hungry in the morning because we've eaten too much in the evening) and gradually bring the time forward so as not to eat it too late. She can do the same with her bedtime. These changes can be made gradually, in quarter-hour increments. Finally, we advise her to maintain her bedtime on weekends, to reinforce her internal biological clock.
For people with a morning chronotype who suffer from waking up too early in the morning and not being able to get through the evening, there are ways of resynchronizing themselves. In addition to general advice, it will be beneficial for them to expose themselves to natural light in the late afternoon or evening, not to bring forward their bedtime excessively and not to neglect dinner, even if it must remain rather light for quality sleep.
Respecting our circadian rhythm is crucial to our overall well-being. By aligning our sleeping, eating, exercise and light exposure habits with this natural rhythm, we promote better mental and physical health. This not only helps prevent certain disorders, but also optimizes our energy, mood and performance on a daily basis. Adopting a lifestyle that respects this rhythm is a fundamental pillar of a healthy, balanced life.
SOURCES :
- https://www.inserm.fr/dossier/chronobiologie/
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