Circadian or nycthemeral biological rhythm: a sleep-wake cycle
To be in harmony with the rhythms of the cosmos — rotation of the Earth on itself and around the sun, the rotation of the moon around the earth — the biology of living beings is organized around sleep-wake cycles which are regulated by a internal clock called biological rhythm.
“In humans, the main biological functions (sleep, heart rate, blood pressure, memory, cognitive abilities, hormonal secretions, food, etc.) fluctuate over 24 hours and are modeled on the day/night cycle, it’s called the circadian rhythm,” says Dr. Ogrizek.
The word circadian comes from two Latin words: “circa” which means “near” and “dien” for “day”.
Dr. Pascale Ogrizek, sleep specialist: Our internal clock, which allows us to respect this circadian rhythm, is located in the brain, in two tiny cerebral structures called the suprachiasmatic nuclei, which are located where the two optic nerves cross.
These two nuclei each contain a few tens of thousands of neurons which can receive information on the level of ambient light intensity. “It is therefore daylight who acts through optical pathwayslike an internal clock re-synchronizer, which needs to be synchronized regularly to operate on precise 24-hour cycles” she adds.
When re-synchronization cannot be done on a regular basis — as is the case for people who are never subject to daylight — biological cycles can fluctuate between 11:30 p.m. and 12:30 a.m. depending on the person and end up shifting completely.
Melatonin, cortisol: the hormones involved in the circadian rhythm
The two hormones that are involved in the circadian rhythm are:
– Melatonin: This night hormone (whose name comes from the Greek “melanos” which means black) which promotes sleep is secreted at nightfall, with a peak around two o’clock in the morningthe time of deepest sleep and lowest body temperature.
– Cortisol, the awakening hormone, takes over in the early morning and provides energy on waking.
The secretion of melatonin is directly influenced by lightbut only light natural of the day, whose wavelength is specific and different from that of electric light. “Light information is captured by the retina then transmitted by the optic nerves to the supra-chiasmatic nuclei, which in turn transmits the information to the pineal gland responsible for the secretion of melatoninsummarizes Dr. Ogrizek. Exposure to daylight blocks melatonin synthesis while darkness triggers it.
Test: How to know your biological rhythm?
If we are all connected to the circadian rhythm, we do not all have exactly the same biological rhythm. When some are more of a morning person and tend to nose dive quite early in the evening, others are the opposite of real night owls who go to bed at “no time” and have trouble waking up in the morning. .
“This difference in biological rhythm specific to each one is called the chronotypeand is function of the schedule of melatonin secretion“explains the sleep specialist.
Early goers/early risers secrete it immediately as soon as night falls, while night owls/late risers take much longer to secrete it.
This chronotype would be determined on the one hand by genetics, and on the other hand by the environment.
To find out your chronotype, there are several questionnaires relating mainly to preferences for waking and sleeping times as well as the optimal times for performing certain activities. The most famous and used is the ” Morningness-Eveningness Questionnaire established by Horne and Ostberg in 1976. which makes it possible to identify 5 different chronotypes using 19 questions morning type (score of 70 to 86), moderate morning type (score of 59 to 69), no type (score of 42 to 58), moderate evening type (score of 31 to 41) and evening (score of 16 to 30).
Disturbed circadian rhythm: what are circadian rhythm disorders?
If most of the time our internal clock works correctly, certain situations can disturb it and cause arrhythmias.
“Apart from this internal clock which is the conductor of all hormonal secretions, there are also peripheral clocks located in each organ (heart, lung, liver, muscle, kidney, retina) which serve as relays when the environment induces situations where it is necessary to adapt” summarizes the specialist.
This is for example the case of night workers who have need to stay active at night despite the fact that they continue to secrete melatonin since there is no daylight. They must adapt their cardiac, digestive, muscular, respiratory activity, etc. to their nocturnal activity, and this is where peripheral clocks come in.
“Usually these peripheral clocks work autonomously according to a rhythm of their own, but they must be constantly resynchronized by the brain’s internal clock. If this is not the case, they get out of sync and work in cacophony as if there were no longer a conductor: this is where we talk about circadian rhythm disorders” summarizes Dr. Ogrizek .
These disorders are manifested, among other things, by changes in the rhythm of sleep.
There is three major types of change in sleep patterns :
- phase advance : These are subjects who fall asleep very early (around 8 p.m. for example) and wake up very early (4 a.m.). “Phase advance is very common in old people who tend to fall asleep early because they are bored at night, wake up at dawn and sleep during the day” describes Dr Ogrizek.
- The phase delay : These people go to bed very late in the middle of the night and get up very late in the middle of the day. It’s a rhythm very common in adolescents and young adults who use screens to fall asleep.
- The free course : This is what ends up happening when there is no more access to daylight, and therefore no more re-synchronization of cycles. “This is the case with blind people who no longer have any perception of light or speleologists whose cycles last between 11:30 p.m. and 12:30 a.m. without being synchronized, and the rhythm gradually shifts each day until it is completely reversed” sums up the specialist.
Depression, anxiety, cancer and a number of other pathologies can promote a desynchronization of the biological clock.
Dr. Ogrizek: People who work staggered hours often suffer from biological rhythm disorders. Numerous studies have shown that they are also more prone to cardiovascular disease, diabetes, ulcers, depression, cognitive disorders, and women to fertility disorders.
In addition, night work would increase the risk of cancer, and is classified as “probably carcinogenic” by the cancer research center.
Physical activity, diet: How to regulate your circadian rhythm?
There are several ways to regulate a disturbed biological rhythm or to keep it balanced.
– Light exposureespecially in winter, through thelight therapy (or phototherapy) : It consists in exposing oneself to the light of a lamp whose luminous intensity is between 5 and 10 thousand lux, i.e. close to daylight. “People with a late phase will expose themselves in the morning for between 30 and 60 minutes to stimulate alertness. And people with a phase advance will expose themselves more in the evening to delay bedtime,” explains the sleep specialist.
– Physical activity : Doing sports in the morning before starting the working day helps to activate the internal clock, therefore to wake up in order to sleep better at night. If physical activity can only be practiced in the evening, it is recommended to take a lukewarm shower immediately after sport in order to lower the internal temperature of the body and promote sleep,
– Food: Certain nutrients have an influence on awakening and falling asleep. “THE proteins boost alertness and should therefore be preferred in the morning and at noon, whereas carbohydrates promote sleep and should be taken in the evening” summarizes Dr. Ogrizek.
Night workers must therefore avoid snacking on chocolate or biscuits during their break at the risk of a nose dive. A slice of ham, a piece of cheese or an egg will be much more effective in keeping them awake and in shape.
– Melatonin : immediate or prolonged release depending on the type of sleep disorder, can be very effective in improving arrhythmias. The first is an over-the-counter food supplement with approximate dosages and the second is a drug found in pharmacies. Immediate-release melatonin can also be prescribed by a doctor in an extemporaneous preparation for precise dosing.
Finally, all common sense advice for improving sleep is recommended in the event of biological rhythm disorders: healthy lifestyle, regular sleep schedules and avoid screens at least an hour and a half before going to bed. Indeed, the light from screens (as well as that from LED lamps) has the same wavelength as that of daylight and delays or stops the secretion of melatonin, somehow making the brain believe that it is is morning. It therefore prevents you from falling asleep.
Ultradians and Infradians: What are the other two biological rhythms?
The ultradian rhythm is a biological rhythm whose period is shorter and whose frequency is higher than a circadian rhythm: its duration is therefore less than 24 hours. This type of rhythm is present in many phenomena in humans: heart rate, respiratory rate, hormone release, sleep cycles, etc.
The infradian rhythm is a biological rhythm whose period is longer and frequency lower than a circadian rhythm: its duration is more than 28 hours. In humans, the menstrual cycle, which lasts 28 days, is a perfect example of the infradian cycle.