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 itself? How important is it for our health? That's what we'll discover 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 cycles in a living being . Lasting approximately 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 called the suprachiasmatic nucleus (SCN), and by peripheral clocks , located in many tissues and organs (muscles, liver, pancreas, kidneys, lungs…).
It has been shown that this system, which we will summarize as the " internal clock ," has its own rhythm, between 23h30 and 24h30 depending on the individual . To be in harmony with its environment and the day/night cycle, the internal clock constantly resynchronizes itself on a 24-hour cycle thanks to external factors , called " zeitgebers ," and in particular natural light.
In practice, the central clock receives light signals from the retina to synchronize with this 24-hour cycle. Then, it communicates with the peripheral clocks using several pathways :
Neuronal : sending electrical impulses via the autonomic nervous system (the nervous system that regulates the involuntary functions of the body) directly to target organs and tissues.
Hormonal or humoral : regulation of the secretion of hormones, such as cortisol or melatonin, which will act remotely on these peripheral clocks.
If peripheral clocks are influenced by these signals from the SCN, they are also influenced by other zeitgebers (e.g., the presence of nutrients) and communicate back with it.
The circadian system, therefore, allows, in a way, the temporal division of the body's physiological functions in order to optimize their efficiency according to the time of day , which is more conducive to physical activity or recovery. It regulates processes such as digestion and the sleep/wake cycle, and to this end, influences the secretion of hormones depending on the time of day or night . Thus, melatonin , a hormone produced by the pineal gland that signals darkness and participates in the regulation of the sleep-wake cycle, begins its production at the end of the day to facilitate falling asleep and reaches its peak between 2 and 4 a.m. to induce deep sleep (necessary for physical recovery, memory consolidation, and strengthening the immune system), before dropping to almost zero in the early morning to promote wakefulness ; cortisol , for its part, reaches its peak upon waking to stimulate the energy and alertness necessary for wakefulness. Other hormones also influence hunger depending on the time of day.
According to research, almost all biological functions are governed by this circadian rhythm . Simply put, the body organizes itself to know when to digest, when to rest, and when to secrete specific hormones according to the usual rhythm of our internal clock, which acts as a conductor . If this conductor deviates too much from its score—for example, if we sleep during the day or eat at night—it creates chaos. Certainly, our body is well-designed and capable of adapting to changes in our habits. However, when these changes are too frequent, or when these habits are too far removed from our nature as a diurnal species, this adaptive capacity is overwhelmed , and this can affect our health and well-being in the long term . Synchronizing the internal clock with the environment is therefore essential for maintaining good health .
Circadian rhythm disorders: the crucial role of sleep
Certain conditions , such as obesity, type 2 diabetes, mood disorders, sleep disorders, and even some neurodegenerative diseases, have been linked to disruptions in circadian clocks . Sometimes these disruptions are caused by genetic mutations in clock genes, but often they are caused by external factors that can be influenced.
In our modern societies, the lack of exposure to natural daylight and, conversely, exposure to artificial nighttime light , particularly that generated by screens, disrupts melatonin production. This results in delayed sleep onset and therefore leads to a desynchronization of our natural rhythms .
A study published in 1999 in The Lancet showed decreased insulin sensitivity , alterations in TSH secretion (the hormone that stimulates the thyroid to secrete thyroid hormones), and an increase in nighttime cortisol after just six nights of reduced sleep (4 hours per night). Other studies have shown that sleep restriction leads to an increase in white blood cells , primarily neutrophils. Immune function could therefore also be affected by sleep deprivation. Still other studies have observed that sleep restriction leads to an increase in ghrelin concentrations (the hormone that stimulates appetite), a decrease in leptin concentrations (the hormone that controls the feeling of satiety), and an increase in appetite , particularly for fatty and sugary foods . It would therefore also influence eating behaviors.
Some people are obviously more sensitive to the negative effects of sleep deprivation, many factors are involved, and further studies are needed to better understand what is happening, but it is reasonable to think that disturbed sleep can have multiple consequences for health.
Taking care of your circadian rhythm
Bedtime:
Here are several tips if you want to improve the quality of your sleep:
Sleep in a dark, quiet and cool room (around 18°C; 20°C for the elderly).
Go to bed at a regular time, 2 to 3 hours after dinner to allow digestion to finish.
Getting enough sleep , around 8 hours for most people.
Take naps if the need arises, but limit them to 20 minutes and not after 3 p.m.
To avoid rumination and calm emotions, practice breathing exercises . For example, diaphragmatic breathing : lying in bed with one hand on your stomach, feeling it rise as you inhale and fall as you exhale; or heart coherence : 5 minutes three times a day with 6-second inhales and 6-second exhales. Some people will appreciate bedtime stories or relaxation techniques offered by certain apps (notably Natflow ) or by disconnected devices like Zenspire if you prefer to leave your phone outside the bedroom.
The light:
Exposure to light is crucial for proper circadian rhythm synchronization. Here again, there are some best practices:
Exposure to natural morning light is recommended. If this isn't possible, a light therapy lamp can be helpful for those who don't have the opportunity to expose themselves to sunlight regularly. In this case, it's advisable to use a lamp with a power output of 10,000 lux and classified as a medical device (such as those from brands like Beurer or Philips).
Reduce exposure to artificial light , especially blue light emitted by screens such as phones, tablets, and computers. To limit this exposure, for example, for people who work all day on a computer, wearing blue light filtering glasses can be an option. In any case, it is advisable to avoid these screens in the evening and turn them off about two hours before bedtime to avoid disrupting sleep. Some people even replace white light (for example, from their bedside lamp) with red bulbs in the evening, as this color is said to be less stimulating to the brain, not to affect melatonin production, and to help relax the nervous system, promoting restful sleep.
Food:
Nutrients act like timekeepers on the internal clock, which is why it will be important to develop good habits:
Have a light dinner in the evening , do not overload on protein and avoid fast sugars (sugary products) which tend to make you jittery.
Prioritize protein and good fats (such as rapeseed or oilseed vegetable oils) at the beginning of the day to support energy and alertness.
Maintaining regular meal times , even with irregular hours, helps stabilize the biological clock.
Stop consuming stimulants (tea, coffee, caffeinated sodas, etc.) after 4 p.m., or 2 p.m. for those who are more sensitive . Note that for some people, dark chocolate can also be stimulating.
Physical activity:
Engaging in physical activity is beneficial for health, reduces stress, and can help offset the negative effects of a disrupted circadian rhythm. For good sleep, it is advisable to:
Practice your sporting activity during the day. Ideally, do it in the morning while taking advantage of natural light.
Avoid exercising in the evening or opt for gentle activities that don't raise your heart rate or body temperature. For intense activities, it's best to do them before 6 p.m. to avoid disrupting sleep. Activities based on gentle movements and breathing exercises, on the other hand, can improve relaxation and contribute to quality sleep.
Focus on specific chronotypes
There is significant inter-individual variability in circadian rhythms. Some people are described as having an " early bird" chronotype because they tend to get up and go to bed early; others, with an " evening bird" chronotype, get up and go to bed late. Chronotype is determined by genetics, but not exclusively; environmental factors such as light exposure and meal times can also influence it.
For those with an evening chronotype , some general advice may not be applicable. If someone has this chronotype because their professional life, for example, requires them to get up early, they would benefit from trying to resynchronize themselves to the rhythm of their environment. To do this, they can follow the advice mentioned earlier , particularly regarding exposure to natural light in the morning , but gradually. They can start by eating a light breakfast, gradually making it more substantial. Similarly, they can gradually reduce the size of their dinner (sometimes we aren't hungry in the morning because we eat too much in the evening) and gradually move it earlier so it's not eaten too late. They can do the same with their bedtime. These changes can be made gradually, in fifteen-minute increments . Finally, they are advised to maintain their bedtime on weekends to reinforce their internal biological clock.
For individuals with a morning chronotype who struggle to wake up too early and have difficulty staying awake in the evening, there are ways to resynchronize their sleep cycle. In addition to general advice, it will be beneficial for them to expose themselves to natural light in the late afternoon or evening , to avoid going to bed too early, and to ensure they eat dinner, although it should remain relatively light for quality sleep.
Respecting our circadian rhythm is crucial for our overall well-being. By aligning our sleep, 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 daily performance. Adopting a lifestyle that respects this rhythm is a fundamental pillar of a healthy and balanced life.
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Gregory DM Potter, Debra J. Skene, Josephine Arendt, Janet E. Cade, Peter J. Grant, Laura J. Hardie, Circadian rhythm and sleep disruption: causes, metabolic consequences and countermeasures, Endocrine Reviews, volume 37, number 6, December 1, 2016, pages 584–608, https://doi.org/10.1210/er.2016-1083
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