Shine red light on an aching shoulder and it may calm inflamed tissue. Shine that same red light into someone’s eyes during a migraine and it can make the pain worse.
That split is the central finding of a new mini-review in Frontiers in Pain Research, led by Yuxuan Ren and colleagues at the Health Science Center of Ningbo University. The researchers pulled together two branches of light-based pain science that rarely get discussed together: local red and near-infrared LED treatment applied directly to sore tissue, and visual light exposure, where the eyes themselves are the target.
Two kinds of light therapy, two different targets
Local photobiomodulation is the version most people picture when they hear red light therapy: a panel or wand held against a knee, a lower back, or a stiff neck. The light penetrates a few millimeters to a couple of centimeters into tissue.
Visual light exposure works through an entirely separate route. Light enters the eye, hits the retina, and signals travel into brain circuits that regulate pain independent of any effect on the painful tissue itself.
What red light does once it reaches sore tissue
The review traces several overlapping mechanisms for local LED treatment. In animal models of inflammatory pain, the analgesic effect of near-infrared LED exposure was blocked by injecting an opioid-blocking drug directly into the treated paw, but not when the same drug was injected into the spine.
That pattern points to a peripheral opioid effect happening at the site of pain rather than in the central nervous system. The review also describes changes to the L-arginine and nitric oxide pathway, reduced activity of pain-sensitizing enzymes called protein kinase A and C, and lower activation of TRPV1, a receptor found on the C-fibers that carry burning and inflammatory pain signals.
On the inflammation side, treated tissue in the studies cited showed higher interleukin-10, a cytokine that dampens inflammation, alongside more antioxidant enzyme activity and less tumor necrosis factor-alpha. Together, those shifts describe a tissue environment that is less inflamed and less likely to fire pain signals in the first place.
The color that helps a migraine and the one that doesn’t
The visual-light side of the review is where the findings get counterintuitive. Green light activates a set of enkephalin-producing neurons in a brain region called the ventrolateral geniculate nucleus, which projects to the dorsal raphe nucleus and on to pain-modulating circuits in the visual cortex and anterior cingulate cortex.
Preventive exposure to green light for one to two hours a day over more than ten weeks reduced headache frequency, intensity, and duration in people with chronic migraine, according to the clinical studies summarized in the review. During an active migraine attack, green light also produced markedly less pain exacerbation than white or red light.
Red light exposure through the eyes does the opposite. It activates a different, GABAergic population of neurons in the same geniculate region, a circuit the review associates with increased rather than decreased pain, and red light exposure during migraine attacks has been shown to worsen the pain.
White light is more complicated. A brief, bright flash, on the order of 18,000 lux for 30 seconds, activated pain pathways in the studies reviewed, while a dimmer, sustained morning exposure, roughly 3,000 lux for two hours a day over 14 days, reduced pain sensitivity in a formalin-induced hyperalgesia model. Timing and duration, not just color, appear to decide which way the effect runs.
Where this shows up in the clinic
For musculoskeletal complaints, the review cites a wearable device delivering 660-nanometer red light that reduced neck pain and improved range of motion, and a combined 660 and 850-nanometer protocol, applied for 20 minutes three times a week for two weeks, that eased pain from temporomandibular joint disorders. Readers weighing a home device for joint or muscle pain can compare specifics in Athletic Insight’s guide to red light therapy for knee pain, which covers wavelength and dose in more detail.
On the visual-light side, morning bright light exposure of more than 3,000 lux for an hour a day has shown benefit in clinical trials for fibromyalgia and chronic low back pain, conditions where mood and circadian rhythm disruption often compound the physical pain.
The evidence has real limits
None of this is settled science. The field has been burned before: a widely cited 2009 meta-analysis in The Lancet concluded that low-level laser therapy reduced neck pain both immediately and for weeks afterward, but a later reanalysis found the effect largely disappeared once the statistical model accounted for how different the underlying trials actually were.
Ren and colleagues flag a version of the same problem. As they put it, dosimetric heterogeneity complicates interpretation, since duration alone does not define a dose. Two studies using the same wavelength for the same number of minutes can deliver very different amounts of energy if they differ in irradiance, distance from the tissue, and beam area, and few papers report all of those variables consistently.
Small sample sizes and the difficulty of blinding a visual-light study, since patients can usually tell what color room they are sitting in, add further noise to a field that is mechanistically promising but clinically young.
The color matters as much as the device
The practical takeaway is not that light therapy is a coin flip. It is that a red light panel aimed at a stiff knee and a red light shining into tired eyes are, physiologically, two different interventions that happen to share a wavelength.
Until dosing standards catch up with the mechanistic research, the safest read of the evidence is to match the light to the target: local red or near-infrared for tissue, and green rather than red for anything involving the eyes.
