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Green light therapy for humans and colour-blind rats (Member Post)

 •  • by Paul Ingraham
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Weekly nuggets of pain science news and insight, usually 100-300 words, with the occasional longer post. The blog is the “director’s commentary” on the core content of a library of major articles and books about common painful problems and popular treatments. See the blog archives or updates for the whole site.
Photo of through green leaves to bright sky.

If green light is analgesic, surely it has something to do the niceness of chlorophyll.

Green light for headaches and perhaps other chronic pain? Say what now?

Well, it’s not crazy: obviously light can affect us emotionally at the very least. I like sitting in a lush garden as much as anyone. But is that therapy for a headache? Or just “therapeutic,” in the simple sense that any pleasant experience is inherently valuable to any suffering human?

So, here we go again: yet another treatment with hardly any scientific support, but championed by one charismatic researcher who makes a great subject for an NPR article.1 This one is an avuncular and self-deprecating physician, Dr. Mohab Ibrahim, who almost sheepishly claims that he’s merely following the evidence where it’s leading him, even if that is away from his professional comfort zone — “drugs are my tool,” he confesses, as though no one could be more surprised that he’s fallen down this alternative therapy rabbit hole.


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Yet another beguilingly simple, plausible therapy inspired by a “growing body of evidence” … that I cannot actually find.2 That cliché is going to age me prematurely.

Oh, there are papers about phototherapy, no doubt! Fixing bodies with light has always been a popular idea, from infrared to lasers. There’s just precious little that’s actually compelling or hopeful in a clinical context, and that includes all forms of phototherapy.

But there’s just almost nothing at all about green light specifically. How much exactly? Exactly one tiny “preliminary” trial for migraine patients. Thirty subjects! From the lab of the guy who has already obviously decided it works.3 It was positive, of course. Do I trust it? Hardly. We should never trust one study — but especially not tiny preliminary ones with a high risk of bias.

Tell me about the rats

There’s also a bit of rat data that inspired the human trial.4 The rats were allegedly aided, of course, by light from an green LED light, and also by room light filtered through tiny little green rat contact lenses (I am not kidding). Specifically, their pain thresholds were raised (which is not the same thing as treating pain, by the way — related, of course, but not the same).

But we need good human data or GTFO, because “mice lie and monkeys exaggerate” — the biologist’s lament about the unreliability of animal models. And guess what? This study is a perfect example of how rats can lie, because rats don’t see green like we do! Indeed, they barely see green at all — they are like humans with red-green colour blindness. Rat expert: “Their color saturation may be quite faint, and color appears to be far less important to them than brightness.”5 D’oh! Did Dr. Ibrahim even know this? Why would anyone even bother testing green light therapy on colour-blind animals?

Two versions of a flower photo side-by-side: one as a human would see it, with a vivid pink flower surrounded by green foliage, and the other blurry and so desaturated that it’s barely recognizable.

A dramatic re-enactment of how you would see this flower & how a rat might see it — fuzzy & very desaturated.

Do rats actually need to be able to see rich greens to be therapized by green light? Well, um… probably? It’s not impossible that the frequency of light is somehow helpful without being able to perceive it, but it seems super unlikely to me. If there’s a mechanism for green light therapy, it probably depends on actually seeing green. And the study authors obviously thought so too, because they write that their results with the green contacts “argue for a role of the visual system” in pain control.

But rats can barely perceive green, so that seems like a major glitch, and the rat data here may not even be “promising” for rats — let alone humans. Always remember that animal studies can be just as dead wrong as human trials, and they often are.

Is green light therapy a “wake-up call”?

Despite all the limitations of the science, apparently you can still get a money-quote from a person with fancy credentials about how “promising” green light is: “This is a wake-up call,” says Mary Heinricher, another expert quoted by NPR. “There’s something going on there.”

Not until there’s better evidence, there ain’t! There is no “wake-up call” here until we know it actually works. And we absolutely cannot know that without at least a couple larger human trials.

But wait, it gets even sillier, because Heinricher also says: “The effects of the green light is pretty subtle.”

I don’t know about you, but I don’t wake up for “subtle.” So what is it? A subtle effect based on on tiny preliminary human trial? Or a wake-up call? Because it really cannot be both.

The bar for hype is very low these days. Practically anything qualifies.

There is enough here for some harmless curiosity, sure. It’s just lettuce-coloured light. We’re not exactly talking about injecting an insanely potent neurotoxin here (I’m thinking about Botox, having written about that lately). If I had migraines, I’d certainly try it. I’d have a green light in the house just for fun! And I do! If I had some other significant chronic pain problem, I would try it. And I do! So I did.

With results so subtle that I cannot detect them.

Photo of a laptop with a bright green glowing screen, flanked by a pair of green lamps.

Just for fun, I worked for three days with my computer workstation blazing lots of green light at me, plus a couple bright, green LED lamps that I happened to have on hand. I am willing to experiment more, but… well… I actually had more pain in that period than I normally do — including a bad headache!

Other light therapies

I have also written about infrared and laser light therapy — substantive free content on both those topics. I have not written about “red light therapy” specifically, but it is (literally) adjacent to infrared. I’m working my way through the spectrum, ha! 🌈


  1. [Internet]. Stone W. Could Migraine Pain Relief Be Found In The Color Green?; 2023 February 8 [cited 23 Mar 1]. PainSci Bibliography 51232 ❐
  2. Cheng K, Martin LF, Slepian MJ, Patwardhan AM, Ibrahim MM. Mechanisms and Pathways of Pain Photobiomodulation: A Narrative Review. J Pain. 2021 Jul;22(7):763–777. PubMed 33636371 ❐ PainSci Bibliography 51243 ❐ “A growing body of evidence supports the modulation of pain by light exposure,” Cheng et al. writes (a team lead by Ibrahim).
  3. Martin LF, Patwardhan AM, Jain SV, et al. Evaluation of green light exposure on headache frequency and quality of life in migraine patients: A preliminary one-way cross-over clinical trial. Cephalalgia. 2021 Feb;41(2):135–147. PubMed 32903062 ❐ PainSci Bibliography 51240 ❐
  4. Ibrahim MM, Patwardhan A, Gilbraith KB, et al. Long-lasting antinociceptive effects of green light in acute and chronic pain in rats. Pain. 2017 Feb;158(2):347–360. PubMed 28092651 ❐ PainSci Bibliography 51233 ❐
  5. What Do Rats See?” Accessed 2023-02-07.

    This quaint old-school website about rat vision explains in amazing detail. There’s even a “rat cam” that shows “how the world may look through the eyes of a rat”! The file is so ancient that I couldn’t actually get it to work. Not that it’s all that hard to imagine.