Light
Near-Infrared Light
The invisible band just past red — the part of sunlight that penetrates centimetres into tissue, and the basis of photobiomodulation, light used not to see by but to signal with.
Just beyond the deepest red the eye can register, sunlight keeps going. Near-infrared light — roughly 700 to 1400 nanometres — makes up a large share of the energy the sun delivers, and it has a property visible light lacks: tissue is relatively transparent to it. Hold a torch against your palm and the glow that comes through is the red and near-infrared remnant. At these wavelengths, light reaches centimetres deep — into muscle, joint capsule, even bone — which means skin is not the boundary of the body’s light exposure, only its surface.
Photobiomodulation is the use of this band deliberately: red and near-infrared light, from LEDs or low-level lasers, applied at doses far below anything that heats tissue. The best-described mechanism runs through the mitochondria, where the enzyme cytochrome c oxidase absorbs in exactly this window; photon absorption appears to relieve an inhibitory bottleneck, nudging cellular energy production upward and triggering downstream signalling. The fuller story of the cell’s power stations is owed to Mitochondria, an entry still to come.
The state of the evidence
The research field is large, uneven, and genuinely promising in places. The most consistent clinical findings are in tissue repair and pain — oral mucositis, tendinopathies, joint pain, wound healing — alongside a respectable literature in skin health and a younger one in muscle recovery, where pre-exercise near-infrared exposure shows measurable effects on performance and soreness. Dose is the field’s great confounder: too little does nothing, too much does nothing (the response is biphasic), and parameters vary so widely across studies that clean conclusions are hard won.
The library’s wider frame: for most of history, bodies received hours of red-weighted light daily at dawn and dusk. Modern lighting deleted the band entirely — LEDs and fluorescents emit essentially none. Photobiomodulation may be less an exotic therapy than a partial restitution.
Threads through the library
Light’s other great biological role — as the clock’s master signal — is the territory of the Circadian Rhythm, and the rest that rhythm governs is mapped in Sleep Architecture.
Threads