We all know that our bodies are subject to a circadian rhythm, an internal time-tracker that coordinates physiological processes to the 24-hour light-dark cycle of the Earth. As humans, we are diurnal so it makes sense that our bodies respond to changes in UV radiation in order to alter our physiological processes depending on what time of the day or night it is. This is mediated through retinal cells in the eye which feed back the levels of UV to a part of the brain called the suprachiasmatic nucleus.
It is then interesting to know that peripheral tissues such as the skin, liver and adrenal glands are able to maintain circadian rhythms independent of the suprachiasmatic nucleus and even of UV light via so called peripheral circadian clocks or oscillators.
[Mohawk JA, Green CB, Takahashi JS. Central and Peripheral Circadian Clocks in Mammals. Ann Rev Neuroscience 2012; 35:445-462.]
So our skin has a peripheral clock?
Yes, it does. Our skin varies in its ability to recover from injury, maintain hydration, pH and temperature balance and create melanin over the process of a day. Skin cells show autonomous circadian rhythmicity that regulates cell proliferation, DNA repair, and pigmentation and protect against aging and skin cancer development.
[Sandu C, Dumas M, Malan A et al. Human Skin keratinocytes, melanocytes and fibroblasts contain distinct circadian clock machineries. Cellular and Molecular Life Sciences 2012;19:3329-3339.]
[Lubov JE, Cvammen W, Kemp MG. The Impact of the Circadian Clock on Skin Physiology and Cancer Development. Int J Mol Sci 2021;22:6612.]
Keratinocytes
These are the predominant cell type in the skin epidermis. The genes that regulate keratinocyte development are predominantly expressed late at night and early in the morning, suggesting that the skin barrier is strongest in the afternoon.
[Matsui MS, Pelle E, Dong K et al. Biological Rhythems in the Skin. Int J Mol Sci 2016;17:801.]
Melanocytes
These are the skin cells that produce melanin, the pigment that absorbs UV radiation and protects the skin from damage. There is a light-sensitive protein in melanocytes called opsin 4, the levels of which are under circadian rhythm regulation.
The efficiency of DNA damage repair mechanisms in the skin also has diurnal variations with the repair processes being more active during the day. This makes sense as UV light exposure is during the day and reduced at night.
[Innominato PF, Levi FA, Bjarnason GA. Chronotherapy and the molecular Clock: Clinical Implications in Oncology. Adv Drug Deliv Rev 2010;62:979-1001.]
DNA repair
Cells repair DNA more efficiently at night. During the day when UV radiation is at its peak, cells accelerate proliferation instead.
Interestingly, disruptions in circadian rhythms such as those caused by shift working, has been associated with an increased risk of skin cancer, especially melanoma. It is thought that the incidence of melanoma is associated with reduced expression of the clock genes which regulate cell cycles, DNA repair and programmed cell death.
[Castejon-Grinan M, Cerdido S, Sanchez-Beltran J et al. Melanoma-asassociated melanocortin 1 receptor variants confer redox signaling-dependent protection against DNA damage. Redox Biol 2024;72:103135.]
Sunburn
There is also a diurnal difference in when you get sunburnt. UVA radiation in the morning causes more redness than those exposed in the evening.
[Guan L, Suggs A, Ahsanuddin S et al. Circadian Rhythm and UV-induced skin damage: An In Vivo Study. J Drugs Dermatol 2016;15:1124-1130.]
Skin barrier function
There is also diurnal variation in skin barrier function, namely in something we call Transepidermal Water Loss (TEWL) with an increase in skin permeability in the evenings and at night than in the morning. Skin blood flow is also highest in the afternoon and early evening and low in the morning.
[Yosipovitch G, Sackett-Lundeen L, Goon A et al. Circadian and Ultradian (12h) Variations of Skin Blood Flow and Barrier Function in Non-Irritated and Irritated Skin – Effect of Topical Corticosteroids. JID 2004;122:824-829.]
Skin microbiome
There has also been some data to show that a consistently late bedtime can decrease skin hydration, skin firmness and elasticity, increase TEWL, wrinkle formation and sebum formation and also reduce facial skin microbial diversity.
[Shao L, Jiang S, Li Y et al. Regular Late Bedtime Significantly Affects the Skin Physiological Characteristics and Skin Bacterial Microbiome. Clin Cosmet Investig Dermatol 2022;15;1051-1063.]
What does this mean to me?
It could mean that the timing of application of treatment to the skin could improve their therapeutic effect, ie applying a topical steroid for eczema to the skin in the evening will improve and give a more rapid effect than applying it in the morning.
Similarly, if you were to apply a moisturiser only once a day, the diurnal variation would imply that applying it in the evening when the skin is at its most permeable would be more advantageous.
From a skin cancer prevention point of view, being more careful with UV exposure in the morning might mean there is less risk of sunburn.
And going to bed early now has scientifically been proven to improve how you look in the morning!
Kind regards,
Sandy
Dr Sandy Flann, Consultant Dermatologist
