The science of red light.

Red light at roughly 600–700nm and near-infrared light at 700–1100nm penetrate skin and are absorbed by cytochrome c oxidase (CCO), the fourth enzyme complex in the mitochondrial electron transport chain. The seminal mechanistic work by Tiina Karu in the 1980s and 90s — later extended by Michael Hamblin's group at Harvard/Wellman — showed that this absorption displaces inhibitory nitric oxide from the enzyme, accelerating electron flow and ATP production.

The downstream effects cascade. More ATP per cell, transiently increased reactive oxygen species (a signalling boost, not damage at therapeutic doses), and activation of the Nrf2 antioxidant pathway. Fibroblasts proliferate faster. Inflammation markers fall. Stem cells mobilise. The same mechanism, deployed in different tissues, drives the skin, hair, recovery, brain and pain literatures.

This is why dose matters more than people realise. The Hamblin biphasic curve (Anders et al. 2015) shows that benefits rise with dose up to a point — typically around 4–60 J/cm² — then plateau or reverse. More red light is not unboundedly better. The protocols section explains how to find the right dose for your goal.

Lanzafame et al. (2013, Lasers in Surgery and Medicine 45(8): 487–495) randomised 44 men with androgenetic alopecia to a 655nm helmet device or a sham device, used 25 minutes every other day. At 16 weeks, the active group showed a 35% increase in hair count versus controls. A parallel 2014 trial in women (Lasers Surg Med 46(8): 601–607) reported similar improvements.

Subsequent meta-analyses (Afifi et al., 2017; Liu et al., 2019) have confirmed the effect across roughly 20 trials. The mechanism is dual: photobiomodulation extends the anagen (growth) phase of the hair follicle cycle, and improved follicular blood flow / mitochondrial output supports the metabolically demanding hair-growth process.

Practically, the protocol that works in the trials is roughly 4–6 J/cm² per session, two to three times per week, for at least 16 weeks. Beds, helmets, and panels can all deliver effective doses if positioned and timed correctly — see the wavelengths and protocols pages.

Posten et al. (2005, Dermatologic Surgery 31(3): 334–340) reviewed the LLLT wound-healing literature and reported a 40% acceleration in healing rates across the included trials, particularly for diabetic foot ulcers and venous leg ulcers. Subsequent RCTs and meta-analyses have replicated the direction of effect across burns, surgical wounds, and post-surgical recovery.

Mechanism: fibroblast proliferation, collagen synthesis (as in the skin literature), angiogenesis via increased VEGF expression, and modulation of the inflammatory phase of healing. The wound moves from inflammation to proliferation faster and exits cleaner.

This is also why red light is increasingly used in dental practice (Borges Magalhães et al. 2019 on post-extraction healing), in oral mucositis from chemotherapy (Bjordal et al. 2011 systematic review), and in post-laser-resurfacing protocols in dermatology.

Naeser et al. (2014, J Neurotrauma 31(11): 1008–1017) ran an open-label pilot in 11 chronic TBI patients using transcranial LED therapy (633 + 870nm) three times weekly for 18 sessions. Significant improvements were seen in executive function, verbal memory, and PTSD symptoms. Gonzalez-Lima's group at UT Austin has since extended this work to healthy young adults, showing acute improvements in attention and executive function after a single 8-minute forehead exposure (Barrett & Gonzalez-Lima 2013).

More recent work has explored Alzheimer's and major depressive disorder. Cassano et al. (2018, J Psychiatr Res 99: 70–78) reported antidepressant effects from transcranial PBM in MDD patients. The Tedford group has demonstrated 808nm light penetration through human skull (roughly 3% of incident photons reach 4cm depth — small but biologically meaningful).

This is an active research area rather than established practice. The signal is consistent enough that R1SE includes the dose, geometry and risks in our protocols section, but the brain-PBM literature is younger than the skin and recovery literatures. Honesty: the science is exciting and incomplete.