Image by Sammy-Williams from Pixabay
Circadian rhythms represent the physiological, metabolic and behavioral oscillations that the human body undergoes during 24 hours. It is the clock that our body has to modulate the different changes it undergoes throughout the day.
These rhythms are controlled by a series of mechanisms that influence every organ in the body, including the skin.
The Suprachiasmatic Nucleus: the clock that controls rhythms
The “main clock” that modulates circadian rhythms is located in the brain, specifically in the suprachiasmatic nucleus (NSQ). This organ regulates the hormonal levels that allow circadian oscillations.
The clearest example of how NSQ regulates circadian rhythms through hormonal levels is the control of the sleep/wake cycle:
- When there is no light, sunlight or artificial, NSQ induces the production of melatonin by the pineal gland, these high levels of melatonin help you fall asleep.
- Whereas when light hits the retina and optic nerve, this production stops and the body enters the waking phase, waking us up from sleep .
In addition to the production of melatonin, NSQ initiates hormonal and neuronal signals that coordinate other physiological oscillations. In this way, the peripheral organs are functionally synchronized through autonomous clocks present in different cell types, which drive the expression of some genes .
One of these peripheral organs is the skin, which has endogenous rhythmicity and contributes to the circadian rhythm. But how do these rhythms work on the skin?
Circadian rhythms in the skin
The NSQ communicates with the skin through a combination of sympathetic innervation and hormones. In addition to NSQ, the circadian system is made up of peripheral oscillators present in various cell types, including epidermal cells .
In the case of the skin, the circadian clock focuses on the activity of certain genes. In this way, circadian variations in the skin influence transepidermal water loss, proliferation of keratinocytes, blood flow in the skin or its temperature .
Under normal conditions, many properties of the skin follow a periodicity. In this way, during the day; transepidermal water loss (TEWL) is lower, so is blood flow and keratinocyte proliferation rate. While increasing sebum production, temperature and pH levels [5,6]. On the other hand, the skin barrier becomes more permeable at night, this increases the loss of transepidermal water and that, together with the increase in blood flow, increases the sensations such as itching and skin irritation [7,8 ].
In contrast, cell division and DNA repair and replication in the skin occur during the diurnal cycle. These rhythms impact on the immediate (erythema, DNA damage, and immunosuppression) and long-term (skin cancers and photoaging) damage caused by ultraviolet radiation .
Dysregulation of the circadian rhythm and its effects on the skin
Many of the physiological processes of skin cells are controlled by circadian rhythms. Over time, cell proliferation and tissue repair decline, favouring cell senescence and skin aging.
In addition to modulating cell proliferation, the circadian clock also controls the regulation of cell senescence. This leads to various processes associated with aging, such as reduction in skin thickness, delayed wound healing, hair loss, and greying .
These physiological processes associated with skin aging are increased with the dysregulation of the circadian rhythm over a long period of time. In a comparative study conducted between day and night shift workers, it was found that the expression of certain key proteins of the circadian clock is altered. This alteration is related to variations in the properties of keratinocytes and stem cells of the hair follicle, affecting the regenerative properties of human skin and hair precursor cells, which contributes to skin aging .
In addition to aging, the skin has repair mechanisms associated with circadian rhythms. Thus, the repair of DNA damage produced by solar radiation in epidermal cells is greater at night , since the oxidative damage produced in the DNA of epidermal cells produced by solar radiation follows a circadian rhythm. Being lower during the first hours of the morning than in the hours of greater solar radiation. This suggests that during the first hours of the morning, the body performs better DNA repair and that with optimal sleep, this repair is greater .
Therefore, a correct circadian rhythmicity and, especially, having a correct rest during the night, is important to facilitate the skin repair processes, what is called beauty sleep.
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