Skins internal biological clock: circadian rhythms

Circadian rhythms are biological processes linked to the cycles of the day. Many bodily functions vary according to these rhythms, including the following:
▌Body temperature
▌Pulse rate and blood pressure
▌Reaction time and performance
▌The production of melatonin, serotonin and cortisol
▌Intestinal activity
Travellers who make frequent long-distance flights often have direct experience in the importance of getting acclimated to a new time zone. One’s inability to adjust can lead to sleeping problems and disturbances in cognitive functions. People who do shift work, or work under bright lights, can face similar issues. Problems arise whenever the daily rhythm is disturbed. Human beings have an internal clock that lasts about 25 hours and resets itself daily when it is exposed to daylight. Blind people can thus have sleeping problems, and yet, even without the ability to see sunlight, their bodies function mostly just fine. Light clearly has a central role in the regulation of our daily lives, and can be used to reset our circadian rhythms. 

SKIN & CIRCADIAN RHYTHMS
Our skin follows a natural daily cycle, known as the circadian rhythm, which influences its functions at different times of the day. This rhythm helps the skin protect itself during the day and repair itself at night.

Daytime: protection and vigilance
During the day, your skin protects you from various environmental threats, such as harmful UV rays and pathogens. Thanks to the circadian rhythm, your skin's barrier becomes stronger, and its immune defences are on high alert. This means your skin is busy producing protective proteins and ramping up immune responses to keep everything in balance and prevent damage. Activities like cell growth and movement are more pronounced during the day, helping to maintain and repair your skin.

Nighttime: repair, regeneration, and weaker barrier
As night your skin switches to repair mode, focusing on fixing any damage it endured during the day, such as UV-induced DNA damage. The skin cells, particularly keratinocytes, follow a natural rhythm that boosts nighttime repair activities, including increased cell growth and improved barrier recovery. Clock genes like BMAL1 and PER play a vital role in timing these repair processes. During this repair phase, the skin's barrier becomes weaker:
Slower barrier recovery: The skin takes longer to recover from any daytime damage or stress, leaving it more vulnerable.
Higher permeability: While this allows skincare products to penetrate more deeply, it also means the skin is less effective at keeping out harmful substances and Transepidermal Water Loss (TEWL) is increased, meaning the skin is prone to lose more moisture and become dehydrated. 
Disruptions to this rhythm can impair skin function and accelerate aging, highlighting the importance of using the right nighttime skincare products to support the skin's barrier and hydration.
 
The effects of blue light on circadian rhythms: A controversial topic
Blue light, particularly in the 460-480 nm range, has long been considered a potent modulator of circadian rhythms. This short-wavelength light is abundant in sunlight and is also emitted in a very low dose by many electronic devices. The traditional view holds that exposure to blue light, especially in the evening, can disrupt circadian rhythms and negatively impact sleep quality.
Traditional perspective
Research has shown that blue light is particularly effective at suppressing melatonin production, a hormone crucial for regulating sleep-wake cycles [1]. Studies have demonstrated that exposure to blue light can phase-shift the human circadian clock more effectively than other wavelengths [2]. This has led to recommendations to limit blue light exposure from electronic devices before bedtime.
Challenging the consensus
However, recent research has challenged this established view. A study by researchers at the University of Basel suggests that the color of light may not significantly affect circadian rhythms [3]. Instead, they propose that the overall brightness of light plays a more significant role in influencing the internal clock.

Sunscreen and skin circadian rhythms 
There is no direct information about the impact of sunscreen on circadian rhythms. UV radiation, which sunscreen blocks, can affect circadian rhythms. A study on keratinocytes showed that UVB radiation can suppress several genes involved in circadian rhythm regulation for up to 24 hours [4]. The skin has its own peripheral circadian clock [4]. While sunscreen protects against UV (and some sunscreens defend against blue light) damage, it's unclear if it directly affects this skin-specific circadian rhythm. I would consider a significant impact very unlikely, however am curious to see this backed up by scientific research. 

It's clear that light exposure, particularly its timing and intensity, plays a crucial role in regulating circadian rhythms and that circadian rhythms impact our skin and highly recommend daily use of sunscreen with UV protection and blue light defence.

Take care!
Anne-Marie

References:
[1] Ksendzovsky, A. et al. (2017). Clinical implications of the melanopsin-based non-image-forming visual system. Neurology, 88(13), 1282-1290
[2] Tosini, G. et al. (2016). Effects of blue light on the circadian system and eye physiology. Molecular Vision, 22, 61-72.
[3] Spitschan, M. et al. (2023). Effects of calibrated blue–yellow changes in light on the human circadian system. Nature Human Behaviour
[4] Hettwer, S. et al. (2020). Influence of cosmetic formulations on the skin's circadian clock. International Journal of Cosmetic Science
[5] Desotelle, J. A. et al. (2012). The circadian control of skin and cutaneous photodamage. Photochemistry and Photobiology, 88(5), 1037-1047.

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