In the first three articles, we went deep on the breathing environment — air, especially the PM2.5 that is everywhere. Today, we shift our gaze away from the air and toward the energy pouring down on us through window glass, phone screens, and the light tubes above our heads. Welcome to Chapter Two of the Prime Life environment series: light.
“Light Hunger” and “Light Pollution”
Light does far more than illuminate the world. It is a series of precise physical signals that directly regulate our gene expression, protein synthesis, and metabolic rhythms. Through hundreds of thousands of years of evolution, we have become able to derive health benefits from nearly every part of sunlight’s spectrum 1,2.
But modern civilization has placed us in an absurd state: during the day, we hide inside steel and concrete, while outdoor light intensity can be dozens or even hundreds of times stronger than the light indoors, and we suffer from “light hunger”; at night, we are bombarded by blue light from LED screens everywhere, and we live under “light pollution.” This mismatch is an important driver of metabolic dysfunction, emotional decline, and premature aging in modern life 2,3,4.
We touched on light in both the exercise and sleep sections before, but in a scattered way. Over the next few pieces, I want to take it apart more systematically and more deeply.
Today is the first piece — the overarching framework.
Sunlight: Nature’s Perfect Paper
Sunlight is a perfect paper with a complete spectrum. Under the force of evolution, humans can derive benefits from nearly every part of that spectrum (with the exceptions of UVA and UVC). For example —
- Ultraviolet B (UVB)is a temperamental “supplement factory.” It comes with a risk of sunburn, but it is also a crucial key for skin-based vitamin D synthesis and immune regulation [5–7];
- Blue-violet light (450–480 nm)is the brain’s “master conductor of rhythm.” By suppressing melatonin, it kicks you cleanly out of your dreams first thing in the morning and starts the metabolic engine for the day 8,9;
- Green light (500–565 nm)quietly holds the center position in visual perception, allowing you to see the details of the world in all its colors 10;
- Red light (630–700 nm)acts as a “tissue-repair switch” at the cellular level. It provides a kind of gentle biochemical comfort, helping reduce inflammation and accelerate collagen regeneration, while also offsetting part of the damage ultraviolet light does to the skin [11–13];
- Near-infrared light (700–1400 nm)is nature’s “mitochondrial signal lamp.” It has the strongest tissue penetration and can reach deep into cells to “ignite” the energy factories [13–15].
If you ate nothing but carbohydrates every day and never ate protein, you would become malnourished.
By the same logic, if all you receive each day is the incomplete, stripped-down spectrum of indoor LED lighting, your biological system can also fall into a long, grinding state of “light malnutrition.”
Different parts of the light spectrum mainly reshape our bodies through two pathways: the retina and the skin.
Here is a very rough but intuitive way to think about it:
- Blue-violet lightandgreen lightprimarily travel through the retina to the brain, calibrating the circadian clock, regulating hormone secretion, and giving us the ability to perceive color and brightness.
- Ultraviolet light,red light, andnear-infrared light, by contrast, penetrate the skin barrier directly and act on cells — either violently breaking down DNA and collagen (ultraviolet light), or precisely activating mitochondria and accelerating tissue repair (red light and near-infrared light).
The First Pathway: From Retina to Brain
Inside our eyes is a special group of cells called ipRGCs. They are not responsible for helping you “see” beautiful scenery. Their one KPI is to detect light intensity and report directly to the brain’s master clock — the SCN in the hypothalamus: “It’s morning. Time to get to work.”
That is why the first burst of strong light in the morning is the signal flare that activates whole-body metabolism. It directly links up with the body’s four core hormones, like an exquisitely conducted symphony [16–18]:
First, melatonin exits the stage. A strong light signal puts an immediate brake on pineal secretion and clears the residual sleepiness still lingering in the brain.
At the same time, cortisol steps in to take over. It is your natural alarm clock, determining how efficiently you wake up in the morning and how much stress you can handle. This “cortisol awakening response” is like injecting the first shot of fuel into the engine.
Then comes dopamine. Sunlight makes you more alert, which may be related to improved dopamine release, giving you stronger drive and better executive focus.
After that comes serotonin. This is the molecule that helps you feel calm and content, and its available supply depends to a large extent on how much bright light you get during the day.
And serotonin is also the direct precursor of melatonin. The more serotonin you “stock up on” during the day, the more raw material the brain has to synthesize melatonin late at night. So if you do not get enough light exposure during the day, nighttime melatonin production may also be insufficient, and sleep can suffer.
In other words, light becomes the most important calibrator of circadian rhythm through this retinal pathway. But circadian rhythm is about far more than sleep. It is more like a foundational scheduling algorithm: through the rise and fall of these hormones, light regulates your metabolic tempo, immune activity, and even your mood [19–21].
The Second Pathway: From Skin to Cells
As a high-energy particle signal, light can penetrate the skin directly and trigger a cascade of biochemical reactions.
The tradeoff here is fascinating.
On one hand, we face the brutality of light-induced damage. UVA is like a stealth assassin: it penetrates deep into the dermis, violently cuts through collagen, and accelerates aging. UVB, by contrast, is a reckless destroyer, directly bombarding DNA and causing redness and peeling 22.
But on the other hand, the skin is also a vitamin D synthesis factory 23. UVB may be carcinogenic, but it is also the key trigger for making vitamin D beneath the skin. That creates a classic tension: is it really worth accepting DNA damage just to synthesize that bit of vitamin D? We will go much deeper into that question in a later piece.
And finally, we have to talk about the almost miraculous effects of red light and near-infrared light. They penetrate powerfully and can go straight to the mitochondria inside your cells. By activating cytochrome c oxidase (CCO), they can knock away nitric oxide (NO), which inhibits energy production — like clearing a traffic jam — so the mitochondria can accelerate ATP production, the cell’s energy currency. Even more intriguing, red light also acts as a kind of preconditioning signal: it can trigger the skin’s antioxidant defenses in advance and help offset part of the damage caused by ultraviolet exposure 24.
A Quick Summary
Building Your “Light Protocol”
Light’s influence on the human body is a precise and long-term negotiation. It is both a “supplement” that repairs cells and a “flame” that scorches DNA; both a switch that awakens energy and an interference wave that disrupts sleep.
This piece lays out a framework we can keep building on in future research: the effects of different light frequencies on the body, and the two pathways through which those effects are delivered — the retina and the skin.
In the next few installments, we will unpack these points one by one and rebuild a low-cost, high-return personal light protocol — so we can better absorb this life-giving energy that has traveled across billions of kilometers to nourish all living things.
Intervention recommendations are geared toward generally healthy individuals. If you have existing health conditions, always follow your doctor’s instructions.
References
[1]Do MTH, Yau KW. Intrinsically photosensitive retinal ganglion cells. Physiol Rev. 2010 Oct;90(4):1547–81.
[2]Blume C, Garbazza C, Spitschan M. Effects of light on human circadian rhythms, sleep and mood. Somnologie (Berl). 2019 Sep;23(3):147–156.
[3]Brown TM, Brainard GC, Cajochen C, Czeisler CA, Hanifin JP, Lockley SW, et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLoS Biol. 2022 Mar 17;20(3):e3001571.
[4]Burns AC, Windred DP, Rutter MK, et al. Day and night light exposure are associated with psychiatric disorders: an objective light study in >85,000 people. Nat Ment Health. 2023;1:853–862.
[5]Holick MF, Chen TC, Lu Z, Sauter E. Vitamin D and skin physiology: a D-lightful story. J Bone Miner Res. 2007 Dec;22 Suppl 2:V28–33.
[6]Schwarz T. Mechanisms of UV-induced immunosuppression. Keio J Med. 2005 Dec;54(4):165–71.
[7]D’Orazio J, Jarrett S, Amaro-Ortiz A, Scott T. UV radiation and the skin. Int J Mol Sci. 2013 Jun 7;14(6):12222–48.
[8]Brainard GC, Hanifin JP, Greeson JM, Byrne B, Glickman G, Gerner E, et al. Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor. J Neurosci. 2001 Aug 15;21(16):6405–12.
[9]Thapan K, Arendt J, Skene DJ. An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans. J Physiol. 2001 Aug 15;535(Pt 1):261–7.
[10]Mustafi D, Engel AH, Palczewski K. Structure of cone photoreceptors. Prog Retin Eye Res. 2009 Jul;28(4):289–302. Schnapf JL, Kraft TW, Baylor DA. Spectral sensitivity of human cone photoreceptors. Nature. 1987 Jan 29-Feb 4;325(6103):439–441.
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[13]Horton L, Brady J, Kincaid CM, Torres AE, Lim HW. The effects of infrared radiation on the human skin. Photodermatol Photoimmunol Photomed. 2023 Nov;39(6):549–555.
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[16]Gooley JJ, Chamberlain K, Smith KA, Khalsa SBS, Rajaratnam SMW, Van Reen E, et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. J Clin Endocrinol Metab. 2011;96(3):E463-E472.
[17]Clow A, Hucklebridge F, Stalder T, Evans P, Thorn L. The cortisol awakening response: more than a measure of HPA axis function. Neurosci Biobehav Rev. 2010 Sep;35(1):97–103.
[18]Petrowski K, Schmalbach B, Niedling M, Stalder T. The effects of post-awakening light exposure on the cortisol awakening response in healthy male individuals. Psychoneuroendocrinology. 2019 Oct;108:28–34.
[19]Do MTH, Yau KW. Intrinsically photosensitive retinal ganglion cells. Physiol Rev. 2010 Oct;90(4):1547–81.
[20]Blume C, Garbazza C, Spitschan M. Effects of light on human circadian rhythms, sleep and mood. Somnologie (Berl). 2019 Sep;23(3):147–156.
[21]Plano SA, Casiraghi LP, García Moro P, Paladino N, Golombek DA, Chiesa JJ. Circadian and Metabolic Effects of Light: Implications in Weight Homeostasis and Health. Front Neurol. 2017 Oct 19;8:558.
[22]D’Orazio J, Jarrett S, Amaro-Ortiz A, Scott T. UV radiation and the skin. Int J Mol Sci. 2013 Jun 7;14(6):12222–48.
[23]Holick MF, Chen TC, Lu Z, Sauter E. Vitamin D and skin physiology: a D-lightful story. J Bone Miner Res. 2007 Dec;22 Suppl 2:V28–33.
[24] de Freitas LF, Hamblin MR. Proposed Mechanisms of Photobiomodulation or Low-Level Light Therapy. IEEE J Sel Top Quantum Electron. 2016 May-Jun;22(3):7000417.
