Issue 40 — April 2006

Issue 40
Category		Title		Author
Newsletter		Chronobiology and rhythms of the skin		Annette Mehling

Time affects virtually all aspects of our being and is the basis of the underlying rhythmicity which is typical of our lives. To "tell time," most living organisms use internal timing mechanisms known as "biological clocks", many of which are set according to the solar day. These "clocks" coordinate our physiological and behavioral functions with our environment. They signal when to wake, when to sleep, when to eat, they govern when we to go to work or when to school, etc. The study of these temporal biorhythms has been coined chronobiology. This article will give a short overview on chronobiology and examples of chronobiological effects on skin will be described.



Introduction

Although the awareness of time and its influence on various processes dates back many centuries, chronobiology as a science is relatively new. Within a few years, the study of the cyclic nature of daily rhythms was well underway. Since then, hundreds of different clocks and cycles have been identified sparking a renewed interest in chronobiology. A schematic representation of typical chronobiological rhythms is depicted in Figure 1 and definitions (these can vary somewhat depending on the source) can also be found in the glossary (Table 1). Although some of the terminology of chronobiology is based on counterparts found in physics, biologic rhythms are not as precise and thus the word circa has been added.

Fig. 1: Schematic representation of typical chronobiological rhythms Enlarged version

Table 1: Glossary (Adapted from http://www.aamcc.org/glossary.htm. A more exhaustive list can also be found at this website)
Ultradian rhythm	Biologic rhythm with a period shorter than circadian (less than 20 h).
Diurnal	Day related
Nocturnal	Night related
Nycthemeral	A physiological time unit, 24 hours made up of one day and one night. Circadian: About 24 h. The term describes rhythms with an about 24 h ( > 20 to < 28 h) cycle length whether they are synchronized with a 24 h periodic surrounding or not.
Infradian rhythm	Rhythm with a period longer (by definition > 28 h) than the circadian range; the term includes circaseptan, circatrigintan, circannual, and other rhythms of lower frequency
	o Circaseptan: A rhythm with a period of about 7 ( ± 3) days, which may or may not be synchronized with the calendar week.
	o Circadiseptan: A rhythm with a period of about 14 ( ± 3) days. o Circavigintan: A rhythm with a period of about 20 ( ± 3 days).
	o Circatrigintan or circalunar: A rhythm with a period of about 30 (± 5) days. Includes, in mature women during the time of ovarian activity, the menstrual cycle. The term is preferred to the term "menstrual" because rhythms of this frequency are found in premenarchal girls, postmenopausal women and in men; lunular month.
Seasonal variation	Change in a biologic system brought about by seasonal changes of temperature, light-span, etc, and not observed in the absence of such changes.
Circannual	A rhythm with a period of about 1 year ( ± 2 months), synchronized with or desynchronized from the calendar year.

Definition

Chronobiology is the science of investigating and objectively quantifying phenomena and mechanisms of the biologic time structure, including the rhythmic manifestations of life. Term derived from: Chronos (time), bios (life), and logos (science; http://www.aamcc.org/glossary.htm).

While the underlying science of biological clocks is intricate and complex, our master clocks are most likely set according to the world's most reliable time keeper: the sun. This makes the circadian rhythm, the daily cycle, one of the most pronounced and best researched. The coordinated activity of biological clocks controls our daily, monthly, and seasonal behavioral and/or physiological rhythms. In general, our daily life is structured by 3 different clocks (synchronizers): The solar clock which provides daily light and temperature changes; a social clock which is founded on work, school or other social based factors; and the internal biological clock [1] (Figure 2). Researchers have proposed that our physical and mental well-being is most likely determined by the appropriate phasing of these clocks with recurring, meaningful events in our surroundings [2]. In other words, as long as our internal clocks are working properly and are “in synch” with our lifestyles, we can function normally. Placing the chronotype “night owl” in an environment typical for the chronotype “larks (early birds)” can have a profound negative effect on the emotional physical health of the individual. One aspect that should not be forgotten is that life itself is a biological rhythm and many chronobiological rhythms may be out of phase in the elderly population.

Fig. 2: Input and output mechanisms of the biological clock (adapted from http://www.eurosiva.org/Archive/Nice/SpeakerAbstracts/Lemmer.htm) Enlarged version

Circadian rhythms

Data published from various studies has shown that many skin functions have a circadian rhythm (Figure 3). Various parameters assessed in women, e.g. blood flow, amino acid content and TEWL tend to be highest at night [3]. Sebum production peaks around noon [4, 5]. The pH of the skin varies depending on the tine of day and is lowest at night and highest during the day [6, 7]. Skin temperature of the forearm is higher in the late afternoon and that of the face early in the morning [4]. Studies using self-rating of the effects of cosmetics on skin revealed that the women assessed their facial appearance to be better at 10:00 am than at night and that rejuvenating properties of the cream used was better at night [8]. The beneficial effect was both age-related (best effects found in 25-35 year olds than for younger or older women) and related to skin complexion (better effects for fair skinned than dark complexioned subjects).

Denda et al. investigated the recovery in cutaneous barrier functions as assessed by TEWL measurements 1 h after barrier disruption via tape stripping of volar forearm skin at various time points in the course of a day [9]. Time-dependent variations in the barrier recovery rate were observed with significantly decreased rates found between 20:00 h and 23:00 h in comparison to measurements made at other time points. Peaks in skin surface temperature and the basal transepidermal water loss values were observed at about 03:00 h (33.6° C and 0.30 mg/cm²/h) indicating that the time-dependent differences in cutaneous barrier repair are independent of changes in skin temperature and cortisol levels

Fig. 3: Circadian rhythms in skin reactivity (adapted from: http://circadian-rhythms.chronobiology.biz/ and www.aamcc.org/cap3.htm9218.htm) Enlarged version

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Ultradian rhythms — rhythms happening more than once a day — of various skin parameters have also been reported. Skin capacitance, sebum excretion, skin temperature, transepidermal water loss, and skin surface pH on fixed sites of the face and the volar forearm were measured every 4 h under standardized environmental conditions over a time-span of 48 h. Circadian rhythms were detected for sebum excretion (face), transepidermal water loss (face and forearm), skin temperature (forearm), pH (face), and capacitance (forearm). No circadian rhythmicity was found for the other biophysical parameters. Rhythms with periods of 8 h were found for sebum excretion, of 8 and 12 h for transepidermal water loss (face and forearm), and of 12 h for skin temperature (forearm; [4] Figure 4). In general, these results corroborated those found by Yosipovitch et al. who studied similar parameters on face, forearm, back and shin within a 24 h period although several discrepant findings occurred possibly due to the differences in study design [6]. Skin blood-flow and skin barrier function exhibit both circadian and ultradian rhythms with low cutaneous blood flow early in the day and peaks late in the afternoon and late evening [10].

Fig. 4: Circadian effects of skin based on the plexograms generated by LeFur et. al, 2001. Eight healthy Caucasian women in the luteal phase and an average age of 24 y were studied. Measurements were taken on the forearm and face every 4 h over a 48 h period.
Chronobiological effects found in skin
08:00	Peak TEWL (face and forearm)
08:50	Peak of free salivary cortisol
12:00	Peak skin capacitance (forearm)
12:00	Peak sebum secretion (forehead)
12:00	Trough TEWL (forearm)
12:00	Trough skin temperature (forearm)
16:00	Peak TEWL (face and forearm)
16:00	Peak skin temperature (forearm)
20:00	Highest peak skin capacitance (forearm)
20:00	Trough TEWL (face)
0:00	Trough sebum secretion
0:00	Trough TEWL (face)
04:00	Peak skin capacitance (forearm)
04:00	Trough pH (face)
04:00	Peak skin temperature (forearm)

Recent research has revealed that the internal biological clock is controlled down to the genetic level with the genes fittingly being named “clock” (clk); “period” (per), “frequency” (frq), timeless (tim), double-time (dbt), etc. [11]. Constitutive expression of the clock genes clock and period has been reported in the keratinocyte cell-line HaCaT suggesting that the skin may be an additional pace-maker in circadian rhythms [12]. An interesting discovery was recently made, namely that clock gene expression in a keratinocytes cell-line (HaCaT) is modulated by UV irradiation and therefore has a potential role in the regulatory processes involved in the circadian rhythm [13] . Furthermore, keratinocyte proliferation is affected by melatonin, the levels of which exhibit circadian rhythms influenced by light [14] . These studies give further indication that skin processes follow chronobiological patterns and exposure of the skin to UV-irradiation and/or light may possibly be involved in the circadian rhythm regulation via modulation of clock gene expression.

Circuannual/seasonal rhythms

The climatic and physical environmental conditions change with geographic location and the different seasons. In particular solar irradiation, temperature and humidity have an influence on the integrity and function capacity of the skin (but also on the whole body).

Solar irradiation	Acute effects: sunburn; chronic effect ; seasonal variations may play a role in immune responses; photoaging, vitamin D production, etc.
Temperature	Exposure to heat induces sweating, opening of the pores, often vasodilatation and increased blood flow, etc.; exposure to cold often has the opposite effects
Humidity	Exposure to continual dry conditions (murine model ; < 10% humidity) leads to epidermal proliferation and thickening of the skin [15]

Although few detailed or large scale studies of annual or seasonal variations performed on humans have been performed, the studies published do show that the skin also exhibits seasonal variations.

In a time-course study, various seasonal effects were observed [16]. The melanin index was highest in April and December and was lowest in March and October; the erythema index was peaked in April and November and exhibited a trough in March and July. Both moisture levels and TEWL levels were maximal in July. Moisture levels were at their lowest in April and December whereas average TEWL levels were at their minimum in April and October. The epidermis in males was thickest in October whereas the epidermis in women was thickest in July. The epidermis of both sexes was thinnest in April. A similar effect was observed by Nishimura et al. [17] (cited in Akasaka et al.) who reported the lowest hue values and chroma in males around summer solstice with peaks measurable around vernal and autumnal equinox. These reports indicate that seasonal variation in skin patterns is observable.

The levels of lipid peroxides on the skin surface show an increase beginning in spring (April-June) and peaking in summer (July-Sept), possibly as a result of the exposure of the skin to solar irradiation and the gradual loss of catalase activity within the stratum corneum (Super oxide dismutase is not influenced; [18]. This group developed a method to measure catalase of superoxide dismutase on D-squameR tape strippings and further corroborated these findings. They report that catalase activity is low in the summer and high in the winter.

Skin tends to be more hydrated in summer (sweating increases hydration) and dryer in winter (winter xerosis). The incidence of xerosis is much higher in winter and aged skin and this has been correlated to a decrease in stratum corneum lipids. By using tape strippings of female Caucasians, Rogers et al. [19] were able to show that there is a pronounced seasonal decline in most stratum corneum lipids, facial cholesterol and various fatty acids as well as an increase in corneocyte size in xerotic skin. In a recent study by Nakagawa et al. a decrease in the natural moisturizing factor (NMF), in particular with respect to the potassium and lactate levels, with a concomitant increase in pH and stratum corneum stiffness in winter was observed [20].

Conclusions

Studies on chronobiological effects are having a profound impact on many different fields. Many skin functions exhibit chronobiological patterns and in particular circadian rhythmicity has been studied to a greater extent. Skin seems to be more reactive towards late afternoon and evening than mornings and early afternoon. Temperature, skin barrier function, microcirculation, pain perception, pruritus, sebum secretion, skin pH all exhibit circadian rhythms. Skin of old people can exhibit abnormalities in these rhythms. Gender differences may exist and different ethnic groups may exhibit variations in circadian rhythms. Clinicians and researchers need to take these effects into account when designing clinical studies, developing new drugs and delivery systems, when assessing skin diseases, allergic reactions. Yet chronobiology is not only relevant for medical applications. The design of dermatological compatibility and efficacy tests of products used in cosmetics should take chronobiological effects into account, e.g. depending on the test design, measurements of the reactions of a volunteer should be carried out at the same time of day; variations in test results may depend on time of day or season, acclimatization of volunteers to standard environmental conditions can help to keep variations at a minimum, etc.

References

[1] Roenneberg T, Wirz-Justice A, Merrow M. Life between clocks: daily temporal patterns of human chronotypes. J Biol Rhythms. 2003;18:80-90.

[2] Piggins HD. Human clock genes. Ann Med. 2002; 34:394-400.

[3] Reinberg AE, Touitou Y, Soudant E, Bernard D, Bazin R, Mechkouri M. Oral contraceptives alter circadian rhythm parameters of cortisol, melatonin, blood pressure, heart rate, skin blood flow, transepidermal water loss, and skin amino acids of healthy young women. Chronobiol Int. 1996; 13:199-211.

[4] Le Fur I, Reinberg A, Lopez S, Morizot F, Mechkouri M, Tschachler E. Analysis of circadian and ultradian rhythms of skin surface properties of face and forearm of healthy women. J Invest Dermatol. 2001; 117:718-24.

[5] Latreille J, Guinot C, Robert-Granie C, Le Fur I, Tenenhaus M, Foulley JL. Daily variations in skin surface properties using mixed model methodology. Skin Pharmacol Physiol. 2004; 17: 133-40.

[6] Yosipovitch G, Xiong GL, Haus E, Sackett-Lundeen L, Ashkenazi I, Maibach HI. Time-dependent variations of the skin barrier function in humans: transepidermal water loss, stratum corneum hydration, skin surface pH, and skin temperature. J Invest Dermatol. 1998;110:20-3.

[7] Verschoore M, Poncet M, Krebs B, Ortonne JP. Circadian variations in the number of actively secreting sebaceous follicles and androgen circadian rhythms. Chronobiol Int. 1993;10:349-59.

[8] Reinberg A, Koulbanis C, Soudant E, Nicolai A, Mechkouri M, Smolensky M. Day-night differences in effects of cosmetic treatments on facial skin. Effects on facial skin appearance. Chronobiol Int. 1990;7:69-79.

[9] Denda M, Tsuchiya T. Barrier recovery rate varies time-dependently in human skin. Br J Dermatol. 2000;142:881-4.

[10] Yosipovitch G, Sackett-Lundeen L , Goon A, Huak CY, Goh CL, Haus E. Circadian and ultradian (12 h) variations of skin blood flow and barrier function in non-irritated and irritated skin - effect of topical corticosteroids. J Invest Dermatol. 2004;122:824-9.

[11] Young MW, Kay SA. Time zones: a comparative genetics of circadian clocks. Nat Rev Genet. 2001;2:702-15.

[12] Zanello SB, Jackson DM, Holick MF. Expression of the circadian clock genes clock and period1 in human skin. J Invest Dermatol. 2000;115:757-60.

[13] Kawara S, Mydlarski R, Mamelak AJ, Freed I, Wang B, Watanabe H, Shivji G, Tavadia SK, Suzuki H, Bjarnason GA, Jordan RC, Sauder DN. Low-dose ultraviolet B rays alter the mRNA expression of the circadian clock genes in cultured human keratinocytes. J Invest Dermatol. 2002;119:1220-3.

[14] Hipler UC, Fischer TW, Elsner P. HaCaT cell proliferation influenced by melatonin. Skin Pharmacol Appl Skin Physiol. 2003;16:379-85.

[15] Tagami H, Kobayashi H, Zhen XS, Kikuchi K. Environmental effects on the functions of the stratum corneum. J Investig Dermatol Symp Proc. 2001;6:87-94.

[16] Akasaka T, Yoshida A, Fukuda S, Takeuchi T, Katsuzaki N. Yearly changes in the physiological function of the skin. Environ Dermatol. 2002; 9:1-10.

[17] Nishimura K, Kitada Y, Kaneda Y. Skin colors in the four seasons, J Soc Cosmet Chem Japan 1996; 30:169-175.

[18] Hellemans L, Corstjens H, Neven A, Declercq L, Maes D. Antioxidant enzyme activity in human stratum corneum shows seasonal variation with an age-dependent recovery. J Invest Dermatol. 2003; 120:434-9.

[19] Rogers J, Harding C, Mayo A, Banks J, Rawlings A. Stratum corneum lipids: the effect of ageing and the seasons. Arch Dermatol Res. 1996; 288:765-70. [20] Nakagawa N, S. Sakai, M Matsumoto, K. Yamada, M. Nagano, T. Yuki, Y. Sumida, H. Uchiwa. Relationship between NMF (lactate and potassium) content and the physical properties of the stratum corneum in healthy subjects. J. Invest. Dermatol. 2004; 122:755-63.

Please note that a review on chronobiology entitled “Chronobiology: Biological clocks and rhythms of the skin”, coauthored by Joachim Fluhr (Skin Physiology Laboratory, Department of Dermatology, Friedrich Schiller University, Jena, Germany) has recently been accepted for publication in Skin Pharmacology and Physiology.

Author

Dr. Annette Mehling



Dr. Annette Mehling has been working for Cognis Deutschland GmbH & Co. KG since 2001. She is part of the product safety and regulations department and is responsible for dermatological compatibility and efficacy testing. Dr. Mehling is a trained molecular biologist and obtained her PhD in microbiology at the University of Wuppertal. During her 4-year postdoctoral training at the Department of Dermatology, University of Münster, Germany, she was involved in research in the field of cutaneous immunology.