We all follow a 24-hour daily (circadian) rhythm, controlled by biological clocks – the master one being in the brain.
This clock is responsible for setting the programme for all of our bodily functions, (temperature, hormone release, metabolism, sleeping and cognitive function), to be more efficient at certain times of the day.
To ensure that our skincare solutions are delivering exactly what your skin needs, at exactly the time it needs it, we have spent a great deal of time working with experts in the field of Circadian Rhythm to understand its implications across a 24hr period.
Below, Prof. Gaby Badre, Neurophysiologist, details just some of the cycles and challenges your skin undergoes every 24hrs.
the importance of chronotypes
Behavioral manifestations such as the sleep-wake cycles are also controlled by the clock and it is important to note that unlike normal clocks, our biological clocks are not synchronized and do not keep the same pace.
This means that some of us wake early and are more productive in the morning whereas others have difficulty waking up but are wide awake later in the day when they perform the best.
A person’s chronotype (chrono=time) refers to the time of the sleep period, with the “morningness” or “larks” type most alert earlier and the “eveningness” or “owls” preferring to go to bed late. The chronotypes are determined by specific genes but can be flexible, age can have an impact on them: adolescents are often “owls” with a delayed sleep phase while elderly develop a “lark” behavior with an advanced sleep phase.
control of biological circadian rhythms
Our daily (circadian) rhythms in behavior and physiology are driven by a complex internal timing system, consisting of a hierarchical system of oscillators or clocks. Next to the Master “Biological Clock” in the brain (the suprachiasmatic nuclei in the hypothalamus) there are peripheral clocks in all tissues running usually synchronous with the central one.
The biological clock is tuned to the outside world with the help of environmental stimuli (=zeitgeber) of which light is the strongest (controlling our sleep-wake pattern). There are many others, though weaker entraining signals to the 24 hour rhythm such as social cues, temperature, physical activities, feeding.
There is an important link between the central and peripheral clocks (liver, intestine…) and metabolism. A lesion of the master clock abolishes the daily rhythm of feeding and altering time of food intake can lead to a desynchronisation and be related to the development of metabolic disorders such as diabetes, obesity etc… The central clock contributes to the variation in glucose uptake and insulin release.
your skin’s daily rhythm
Whilst the circadian clock in the brain regulates skin cycles, skin cells also have their own clocks which can function autonomously.
These clocks are an important regulator of skin cell proliferation as well as skin aging.
Many functions and properties of the skin follow a rhythm, which differs between different sites in the body.
Factors (external or internal) which can impact on the clocks can disrupt or entrain the cutaneous rhythms with consequences on skin health. Many external environments having diurnal changes may also reset the clocks (pollutants, temperature, humidity, ultraviolet radiation…). The periodicity of skin properties can affect the impact of cosmetics or other products applied on the skin.
The “clocks” regulate skin repair and ageing.
The epidermal (skin) stem cell creates new cells replacing the aged ones. This proliferation occurs mainly late at night – early morning, when the body is supposed to be at rest and not disturbed. Hence the importance of keeping a sleep-wake schedule.
The skin appears more attractive in the morning – after a good night sleep, probably due in part to the proliferation of new cells at night. Therefore some anti-aging creams (stimulating e.g .collagen production) are recommended for use pre-bedtime.
Stem cells are sensitive to pollutants which may disturb cells equilibrium and proliferation. Environmental pollutants such as tobacco smoke, next to its other deleterious effects on the skin, may impact on the stem cells in the skin. It is wise to restrain tobacco use at night.
Skin repair peaks at night.
skin structure & function
Skin is the interface between environment and the body. It is a “barrier” and the first line of defense against microorganisms and environmental factors.
The skin harbors, in addition to skin cells, glands and hair follicles, various microorganisms (microbiota) as well as many transient immune cell types working to counter the development of infections.
Skin cells are engaged in different activities at different parts of the day hence they are more sensitive to damage at certain times.
The skin barrier is less optimal at night.
-Biological rhythms in the skin. Matsui MS, Pelle E, Dong K, Pernodet N. Int J Mol Sci (2016),17,801.
Skin hydration (water and glycerol…) is necessary to maintain a protective barrier against infection and dehydration.
The water content of the skin has a daily rhythm with minimum loss, through the pores of the skin, in the morning and an increased water loss during the night. Hence the skin is more permeable in the evening and night.
This makes the skin more sensitive to irritants and explains that topical application of creams or medicine is better absorbed in the evening.
Skin water loss is high at night.
-Time-dependent variation of the skin barrier function in humans: transepidermal water loss, stratum corneum hydration, skin surface pH and skin temperature. Yosipovitch G, Xiong GL, HAus E, Sackett-Lundeen L, Ashkenazi I, Maibach HI. J Invest Dermatol (1998), 110, 20-3.
-24-hour rhythm of aquaporin-3 function in the epidermis is regulated by molecular clocks. Matsunaga N, Itcho K, Hamamura K, Ikeda E, Furuichi Y, Watanabe M, Koyanagi S, Ohdo S. J Invest Dermatol (2014), 134, 1636-44.
UV exposure & DNA
Chronic exposure to ultra violet (UV) radiation ages the skin with wrinkling, loss of skin tone – and damages the DNA. Sunburn resulting from exposure to excessive UV light confers an increased risk for melanoma. The outer layer of the skin (epidermis) is most susceptible to UV-induced DNA damage during the day.
Sunburn induced cell death (“apoptosis”) and redness (erythema) are controlled by circadian clocks, maximal after an acute early-morning exposure to UV radiation and minimum following an afternoon exposure.
DNA synthesis and repairs are important for defense against UV damage and skin cancer. In skin cell culture this synthesis was reported to occur mainly in the afternoon and evening.
The same dose of UV radiation induces more inflammation – and DNA damage in the morning than in the afternoon.
complexion & your skin’s appearance
The water content of the outermost layer of the skin (with cells containing keratin, a fibrous protective protein) and the oily surface on the skin are important factors in the appearance and function of the skin. High water content and a low sebum (oil) secretion are considered main features for a good skin.
The brilliance of complexion (skin coloring, luminosity, brightness, transparency…) and texture of the facial skin have been reported to exhibit a daily rhythm with a best appearance in the morning – according to a study peaking around 10 am.
The most beneficial effects of facial creams are often observed in the evenings where it has also been found that skin penetration of cream to treat pain was higher in the evening. Hence the recommendation to apply skin creams at night – expecting a better look of the skin.
Complexion peak in the morning.
Penetration peak in the evening.
sebum; lubricating the skin
There are glands on the skin producing an oily, waxy substance (sebum) which waterproofs and lubricates the skin and hair. Sebum protects also the skin against infections. Enzymes on the skin surface gradually decompose the sebum.
The glands secreting sebum have a day rhythm in oily excretion; many authors demonstrated an excretion peak around midday (12:00-15:00) with the lowest levels in the late evening and early in the morning around 6:00-8:00.
Increase in skin temperature may cause an increase in sebum excretion. Skin temperature is affected by several external and internal factors (blood pressure, blood flow…) as well as hormone levels causing seasonal and daily variations.
Too little sebum may lead to dry, cracked skin while too much sebum may clog hair follicles and promote growth of bacteria, normally found in the skin causing e.g. acne. Too much sebum (“seborrhoea'') also makes face exceedingly shiny – affecting e.g. make-up!
sweating & your skin
Sweat helps the skin by unclogging and cleaning the pores. It also contains a natural antibiotic, “Dermcidin”, that acts against bacteria on the skin’s surface.
Leaving sweat on can lead to heat rash with red bumps. Hence it is essential to wash the skin after sweating.
There is a daily variation of sweating rate which is less at night, sweating begins in the early morning increasing during the day getting greater in the afternoon and evening. Sweating rate follows the skin temperature which is low at night, increasing in the morning and peaking in the afternoon-evening (about 1 degree C higher than in the morning).
Perspiration is lower at night between 2:00-5:00 than it is during daytime.
the daily rhythm of wrinkle appearance
There is a daily periodicity of wrinkle appearance.
In the morning, after a night sleep, the face may swell masking wrinkles. During the day, due to gravity fluid shifts to the lower part of the body reinforcing the appearance of wrinkles in the evening.
It has also been suggested that movements of the face during the day may gradually reinforce appearance of wrinkles from the morning increasing them in the evening.
Hence it is interesting to mask the wrinkles in the evening – moisturizers trap waters in the skin).
The appearance of wrinkles peaks in the evening.
-A study of diurnal variation in wrinkles on the human face. Tsukahara K, Morikawi S, Hotta M, Fujimura T, Kitahara T. Arch Dermatol Res (2004),296, 169-74.
-Dermal fluid translocation is an important determinant of the diurnal variation in human skin thickness. Tsukahara K, Takerna Y, Moriwaki S, Fujimura T, Imokawa G. Br J Dermatol (2001),145, 590-6.
If you experience itchy skin, it is likely to be exacerbated at night, and this can be related to the circadian rhythm and changes in skin physiology and its barrier function which is more permeable in the evening compared to the morning. It has been suggested that water loss and increased skin blood flow may contribute to the increased itching experienced at night e.g. eczema.
There is an increase in skin temperature during the night and itching has been reported to be aggravated by ambient heat.
Nocturnal itching may also have a psychological component, exacerbation due to lack of external stimuli and boredom.
Itch is exacerbated at night - control environmental temperature.
-Nocturnal itch: why do we itch at night ? Patel T, Ishiuji Y, Yosipovitch G. Acta Derm Venereol (2007), 87, 295-8.
-Pruritus. An update. Yosipovitch G. Curr Probl Dermatol (2003), 15, 135-64.
There is a strong diurnal variation in the symptoms and severity of inflammatory diseases.
The production of pro-inflammatory substances shows a diurnal rhythmicity with peak levels during the night and early morning – when the level of cortisol is lowest and melatonin is highest. (Cortisol exerts an anti-inflammatory effect while melatonin was reported to have a pro-inflammatory effect in inflammatory conditions such as rheumatoid arthritis).
Hence, for example, the inflammatory activity of psoriasis varies, highest at night and lowest in the morning and the clinical signs of rheumatoid arthritis are more intense in the morning. Therefore treatment with a low dose of cortisone in the morning will have a good impact.
Disruption of daily rhythms aggravate inflammatory diseases.
physical training & performance
While it is known that regular physical activity is essential for well-being, its impact on circadian clock is less familiar. Training may be used to adjust (“entrain”) the 24-hour daily rhythm to environmental cues. A single physical exercise at night may shift the 24-hour rhythm, delaying it, while exercise early during the waking period may advance it.
Association to bright light – e.g. late night light exposure followed by early morning exercise - will enhance the rhythm shift effect.
Though it has been reported that subjects who exercise in the morning tend to do better, there is no clear evidence that calories are burned more efficiently when exercising at different times of day or if it is better for one’s health.
However biological – and psychological rhythms seem to have an impact on training performance, studies on athletes showed more than 25 % better results during the day, with generally the lowest around 7:00 and peaks in late afternoon – early evening. The best physical training impact on muscle hypertrophy and improvement in strength, seems also to occur in the late afternoon – when body temperature is peaking! A raise in body temperature might have an impact since it has been found to increase energy metabolism.
daily rhythms of hunger and satiety
Humans need energy and nutrients, hence feeding in order to survive.
The biological factors of food intake are hunger and satiety (satisfaction of appetite), sensory perception of the nutrient – taste, smell, appearance, texture..– and palatability (i.e. pleasure when eating with an increase in food intake as palatability increases). Biological rhythms control the balance between hunger, appetite stimulation and food intake.
Hunger and satiety show a clear 24-hour daily rhythm. Intake of food in the morning is satiating while meals late in the day have a lower satiating value. Hunger is highest in the evening (ca. 17:00-21:00) while satiety is lowest. With the decrease of satiety feeling, meal sizes increase over the day resulting in greater intake – in order to anticipate overnight fast ?
During the late night (01:00-05:00) hunger is lowest and satiety highest. This may explain the lack of appetite often reported by night workers.