The nycthemeral rhythm is a 24-hour environmental cycle determined by the alternation of day/night, the transition from natural sunlight to the darkness of night. It synchronizes our internal biological clock but can be upset.
Definition: what is the nychthemeral rhythm?
The nychthemeral rhythm is a environmental cycle which corresponds to a space of time comprising day and nightwhich has an approximate duration of 24 hours, and within which an alternation between light and dark takes place. “The duration of the nychthemeral rhythm remains constant but the distribution between light and darkness varies during the year: the maximum light corresponds to the summer solstice and the maximum darkness to the winter solstice. There are therefore small variations during leap years“, specifies Dr Didier Cugy, somnologist. Throughout the year, we perceive the variations of this distribution: growth and decrease of the diurnal period and/or the nocturnal period.
What is the difference with the circadian rhythm?
The circadian rhythm is a intrinsic biological rhythm : “it is in a way that of the internal clock of a living being“. It takes the form of a cycle equivalent to one day, a little more than 24 hours, which will be readjusted daily by the nychthemeral rhythm. It will allow adapt to the environment and govern certain physiological processes like sleep and food, for example. The proper duration of the circadian rhythm goes depend on the person’s genetics : “a mechanism within our body will define a periodicity, specific to each one. In addition to the circadian rhythm which is approximately 24 hours, there are rhythms with periods greater than 24 hours (infradian rhythm) and others less than 24 hours (ultradian rhythm)“. This periodicity will have a link with certain typologies, such as the morning (“be in the morning”) or evening (“be in the evening“).
Who has a nychthemeral rhythm?
Human beings do not have a nycthemeral rhythm, they have a circadian rhythm which is synchronized with the alternation between natural sunlight (daytime) and darkness (nighttime). The nychthemeral rhythm is therefore an environmental rhythm imposed on living beings. In nature, it is this rhythm that controls the breeding periods of small mammals or even the bird migration periods. “The nychthemeral rhythm has an impact on the laying of hens. Since they need more than 10 hours of light to lay eggs. It will also give the signal to the bears to start hibernation.“, he gives examples. Human beings have the possibility of controlling and/or modifying their environment: of adapting their circadian rhythm to this rhythm. “People with intrinsic rhythms (< 24 hours) are often prone to the development of pathologies related mood disorders such as bipolar disorder“, notes Didier Cugy.
Many organic functions follow a daily rhythm which could be compared to the alternation of day and night, and thus to the nycthemeral rhythm. Body temperature will follow the circadian rhythm (24 hours) specific to each: it is at a minimum at the end of the night around 4-5 a.m. and at a maximum, 12 hours before, around 4-5 p.m. Body temperature also changes with advancing age. In aging, the circadian rhythm is often upset, especially with earlier times to get up and go to bed.
The sleep/wake rhythm is intrinsically linked to the nychthemeral rhythm. Our body adjusts its rhythm to that of the environment: “it’s what makes us sleep at night and awake during the day“. As soon as the day fades, living beings produce melatonin whose role is to manage the rhythms of wakefulness and sleep. This hormone is mainly synthesized at night. The internal clock has sequences which thus function according to this day/night rhythm. A veritable “hormonal circuit”: when you wake up, there is a cortisol spike (anti-stress hormone) and at bedtime, a new spike in TRH (which regulates the thyroid), growth hormone and prolactin. The organization of Toddler sleep is not the same as adult sleep. They have rhythms that are not fixed over 24 hours and mainly determined by their meals. Around the age of 4-6 months, the infant begins to sleep through the night: periods are structured and the periods of sleep lengthen (more than 3 hours in a row). Sleep begins to be meaningful and stalls at night. During the day, there will be a nap period, a period during which the child will need to sleep. From 6-7 years old, the nap usually disappears in our latitudes. “It’s necessary preserve the possibility for children to take naps when they want to sleep“, he insists.
Night work is subject to environmental conditions that are not natural. The processes of arousal, attention and efficiency are mobilized through stimuli (activity, noise and light) in the middle of the night. “The internal clock will have to adjust itself, so as to work as if it were daytime, at 0 a.m. as if it were 8 a.m.“. In this context, the body and the secretions of hormones specific to the circadian rhythm are upset. Some health problems can then appear: loss of fat regulation (metabolic syndrome), increased weight, mood disorders and an increase in cancers (breast and colorectal). “Some people will be more adaptable than others to working nights or shifts. It’s a genetic trait, it’s the body that dictates“. There are methods for adapting to a rhythm of nighttime activities: food rhythm (modulating meals), food balance (rich in proteins for wakefulness and carbohydrates for energy), meal. “In all cases, alcohol consumption should be avoided“he notes. Top athletes are sometimes also forced to adapt due to competitions under other meridians. A practical example: during the 2015 European Cup, the Girondins de Bordeaux were to play their return match in Kazakhstan, 4 days after playing in Lille. The latitude difference between Lille and Almaty (capital of Kazakhstan) was 76° (5 hours in the sun). This shift, in addition to the usual scheduling of matches, was ultimately 9 hours and the time between the two matches less than 96 hours. “We offered players to synchronize with the schedules of Kazakhstan and adapt bedtime/wake up times, training sessions and their mealsin order to to adapt their circadian rhythm to the nycthemeral rhythm from Kazakhstan“, explains Didier Cugy. In the end, the Girondins de Bordeaux drew. A good illustration of the application of the adaptability of our circadian rhythm to the environment.
Thanks to Dr Didier Cugy, somnologist in Bordeaux.
Source: ANSES confirms the health risks associated with night work, 22/06/2016, ANSES