635 nm red light. They say they used a 5 Watt laser for 10 minutes on mice, but that the irradiance was only 72 mW/cm^2. This makes little sense since lasers usually focus light in a small area so one would expect irradiance to be many times _higher_ than 5 Watts. Maybe just a translation error

Opposite effects of low intensity light of different wavelengths on the planarian regeneration rate [December 2019]

A variety of wavelengths were applied to worms, which have a high percentage of stem cells, which improves sensitivity. They found that red (635 nm) increased regeneration after decapitation by 24% while green (520 nm) decreased it by 22%. Other wavelengths had negligible effects. They noted another study which showed enhanced regeneration with near-IR (880 nm), but no effects from 630 nm red light. It is unfortunate that the researchers in this did not include longer wavelengths than 635 nm since they could have confirmed the greater effects from near-IR.

Idea: My impression from skimming a number of studies on red and infrared light is that 810-850nm (near-IR) is generally far more useful than red light at 600-640 nm, or deep red at 660-680 nm. I suspect the reason this is not instantly obvious is that most studies are not actually hitting the upper limits of the beneficial dose response. (Some studies manage to show a decrease in effects with an increase in dose, but the dose is not a linear space since total energy as varied with time is the modulated variable with power usually kept constant.)

At a basic level, near-IR effectively penetrates tissue while visible wavelengths for the most part do not. Given similar power levels being studied with both, there is then a much higher volumetric dose from red since it concentrates on a thin surface. I suspect even larger effect sizes would be possible if the power levels of near-IR were increased correspondingly. (Note, this is a bit of a simplification since there are both localized and global effects.) A key reason this isn't happening is likely ease of use. 620-635 nm light as was used in this study is easy to obtain because it is the wavelength that is used commercially to produce red light. 660 nm is very uncommon because it is a bad choice if your goal is simply to illuminate a room. I cannot be certain about my conjecture, but if I do nothing to determine the answer to this question, that I actually care about, the scientists will be perfectly happy studying thousands more worms and learning all the minutia there is to know in that niche.