Circadian rhythm and sleep: how does it work?
We have a question, which may seem absurd to you: Do you know why we sleep at night?
Because our internal circadian clock synchronizes with the rhythm of the day and the alternating day / night nature of light. But do you know what circadian rhythms are and how they work?
Circadian rhythm: your body’s great daily cycle
First of all, remember that “circadian” means “about a day”. All of the body’s biological processes are subject to a rhythm of around 24 hours, more or less. We can see our metabolism and our body as a complex system, with many cogs, which are regulated by multiple small clocks, themselves controlled by a large internal biological clock. These rhythms are the subject of numerous studies and chronobiology as such is of great interest to research and medicine, in particular to help patients suffering from sleep or mood disorders, the two possibly being often linked.
Thus, our blood pressure, the intensity of our intestinal activity, the temperature of our body, the production of certain hormones (such as melatonin), and even our mood, our memory, our level of alertness and of course our sleep … in short: each of these functions experiences variations during the day, sometimes with activity peaks and breaks.
For example, we know that vigilance is at its maximum in the morning and that it experiences a down in the early afternoon. You must have seen it for yourself: you are more efficient, more concentrated at some times than others.
This rhythm, our internal biological clock, is said to be “endogenous”, that is to say that we produce it ourselves. It is unique to each of us. This regulation takes place in a specific place in our brain: at the hypothalamus, just above the place where your optic nerves cross. This area dedicated to our biological clock is called the suprachiasmatic nuclei (NSC, for short). There are two parts and are made up of tens of thousands of neurons each, which are responsible for sending the right rhythmic instructions to other areas of our brain.
Keep in mind that there is also some genetics involved: the expression of certain genes is partly responsible for your cyclical behavior. The mechanism is of course more complex, but recent discoveries on the subject, for example, in 2017 three Americans won the Nobel Prize in Medicine for a study that demonstrated this genetic role in the circadian rhythm.
What about light?
All this is very interesting, but it still does not answer the age old question of why we prefer sleeping at night rather than at any other time.
The explanation comes from light. As we said, “circadian” means around a day. It’s the “around” that’s important here. Imagine, for example, that your own internal rhythm is set to 25 hours. Without synchronization with the outside world, you would end up completely out of sync of one hour a day. Scientific experiments have demonstrated this effect on people plunged into darkness for a set amount of time: they continued to have a rhythm, but set on their own internal schedule. A famous example is that of Maurizio Montalbini, who, in 1993, spent 366 days in a cave, voluntarily isolated from everything. When he came out he had the feeling that it had only been 219 days.
Fortunately for you, you are exposed to light daily! In your retina, small special cells (Melanopsin retinal ganglion cells or mRGCs), capture this light and send this signal directly into the NSCs, remember, these little nuclei are located just above the optic nerves – how convenient!
Good exposure to daylight is therefore the main “time tellers” for our circadian rhythm. It allows it to synchronize every day, to reset itself every hour.
There are other “time tellers”, such as temperature, but they play a lesser role compared to that of light.
That’s why, therefore, we sleep at night. If we summarize, our eyes, perceiving that the light declines, inform our internal biological clock which sends the signal of light extinction to the entire organism. Which can then explain this famous winter fatigue, period during which the days get shorter and therefore the time of daylight as well.
But what happens if our circadian rhythm is disturbed?
It does happen that our circadian cycle is disrupted, bringing with it its share of sleep disorders, with significant impacts on health:
– the role of light in regulating our circadian rhythm and the involvement in particular of photosensitive melanopsin ganglion cells in our retina, explains why prolonged exposure to blue light from screens at night poses a risk of sleep disturbance
– Graveyard shifts (or night work shifts) is another situation where the impact of our biological clocks on our health is obvious. Those who work at night find themselves out of sync and accumulate a sleep debt. This comes with proven health risks and in particular increases the risk of cancer.
In a more punctual way, you can be confronted with a disturbance of your circadian rhythm and find yourself in a “Delayed sleep phase”: that is to say that your day is shifted. You are ahead of the stage if you get up very early and go to sleep very early. You will be late if you go to bed very late and get up later. Another situation, which you must have noticed during a trip abroad: the famous jetlag effect is one of the expressions of a disturbance of your rhythm.
A good remedy for this is a gradual change of habit that will have to be introduced: going to bed each day a little later in the evening if you are in the early phase, to resynchronize yourself to an acceptable rhythm or going to bed and getting up a little earlier everyday if you are in the late phase. You can also use this solution if you plan to travel to another time zone a early few days before departure, to attenuate the effects of jet lag.
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