therapies · education ·
With Red Light, More Is Not Better. Too Much Undoes The Benefit Entirely
Photobiomodulation has a dose curve that rises then falls. It is why the studies contradict each other, and why a brighter panel can do less.
Almost every wellness intervention gets sold on the assumption that more is better. Bigger panel, longer session, stand closer.
Photobiomodulation is the one where that assumption is not just wrong, it is precisely backwards past a certain point, and there is a named curve for it.
The curve
The finding is called the biphasic dose response, often drawn as the Arndt-Schulz curve. It goes up, reaches a peak, and comes back down.
In plain terms: low levels of light stimulate and repair tissue better than higher levels do (Huang et al., Dose-Response, 2009).
Not “less effective at low doses and more effective at high doses.” Better at low, worse at high. Past the peak, adding more light takes the benefit away.
Why this explains the mess
Here is the part that reframes the whole field, and the reviewers state it plainly.
There are a large number of parameters to choose between: wavelength, fluence, power density, pulse structure, treatment timing. Choosing badly among them is not a small error.
The complexity of rationally choosing amongst a large number of illumination parameters has led to the publication of a number of negative studies as well as many positive ones.
Huang et al., Dose-Response, 2009
Read that against the usual argument about whether red light “works”. If a therapy has a dose peak, then a trial run past that peak will find nothing, and will be right about what it measured and wrong about the therapy.
That is a genuinely different situation from an intervention that simply does not work. It also means you cannot settle this by counting positive and negative studies, which is what most internet arguments amount to.
How deep the pattern goes
An update six years later went further (Huang et al., Dose-Response, 2011).
ATP and mitochondrial membrane potential both show biphasic patterns in cell studies.
Mitochondrial reactive oxygen species show a triphasic response, with two distinct peaks. Not a simple hill at all.
The proposed reason is what the authors call the Janus nature of reactive oxygen species: a useful signalling molecule at low concentrations, and a harmful cytotoxic agent at high ones. Same substance, opposite jobs, depending entirely on how much.
And in transcranial work in mice with traumatic brain injury, the biphasic pattern appeared in two separate dimensions: when the number of treatments was varied, and when the energy density of each individual treatment was varied.
So both how often and how strong have their own peak. You can overshoot either one independently.
Grade that honestly: this is cell work and animal work, not human outcome trials. It is mechanism evidence explaining a pattern, and it is the best explanation anyone has offered for why this literature looks the way it does.
What this means for what you buy
Brighter is not automatically better, and may be worse. A more powerful panel used the same way delivers a higher dose, which on this curve can move you past the peak rather than further up it.
Closer is not automatically better. Distance changes the power density reaching your skin, which is one of the parameters that defines where you sit on the curve.
Longer is not automatically better. Same reasoning. Time multiplied by power density gives fluence, and fluence is the axis the curve is drawn against.
More sessions is not automatically better. The mouse work found a peak in treatment number as well as intensity.
Which leaves the only defensible approach: copy the parameters of a trial that reported a result, rather than maximising anything. The trial we cover in the companion article used roughly 9 J/cm² in the 611 to 650 nm range, twice weekly, for 30 sessions.
What we are and are not saying
| Claim | Our line |
|---|---|
| Photobiomodulation treats, cures or prevents disease | Never |
| The dose response is biphasic | Yes, shown in cell and animal work, and named by the reviewers |
| Negative studies may reflect bad dosimetry rather than no effect | Yes, and the authors say so themselves |
| We know the optimal human dose | No. Nobody does, and the reviewers call for work to establish it |
| Buying the most powerful panel is the safe choice | No. On this curve it is a way to overshoot |
Where the design shows up
There is a version of this in almost everything the body responds to.
Water is required and drowns you. Oxygen is required and is toxic at pressure. Exercise is a stimulus at one dose and an injury at another. Sunlight builds vitamin D and burns you. Even the Brazil nut we wrote about is a nutrient at one a day and a problem at four.
A body is tuned to a range, not to a maximum. The dose curve in photobiomodulation is not an oddity of that therapy. It is the same shape everything else has, made unusually visible because somebody bothered to plot it.
“Have you found honey? Eat only what you need, that you not have it in excess and vomit it.” Proverbs 25:16, NASB
The verse carries the conviction. Two reviews of the dose curve carry the shape.
What to do this week
If you own a panel, stop trying to get more out of it.
Write down what you are actually doing: wavelength, distance, minutes, sessions per week. Then compare that to what a published trial used rather than to what the manufacturer’s maximum allows.
If you are shopping, ask for wavelength and power density at a stated distance. A seller who answers in watts alone is selling you the wrong number.
Read this next: the evidence half of this pair, including the controlled skin trial and the sham-controlled hair trial: red light raised collagen density in 113 people. And the free source of the same wavelengths, with its own trial evidence: morning daylight worked as well as melatonin.
Sources
- Huang YY, Chen ACH, Carroll JD, Hamblin MR. “Biphasic dose response in low level light therapy.” Dose-Response, 2009;7(4):358-383. https://pubmed.ncbi.nlm.nih.gov/20011653/
- Huang YY, Sharma SK, Carroll J, Hamblin MR. “Biphasic dose response in low level light therapy: an update.” Dose-Response, 2011;9(4):602-618. https://pubmed.ncbi.nlm.nih.gov/22461763/
- de Freitas LF, Hamblin MR. “Proposed Mechanisms of Photobiomodulation or Low-Level Light Therapy.” IEEE Journal of Selected Topics in Quantum Electronics, 2016;22(3):7000417. https://pubmed.ncbi.nlm.nih.gov/28070154/
Article metadata retrieved from PubMed. Photograph from Pexels. Scripture quoted from the New American Standard Bible.
Educational content from True Health Solutions. We do not diagnose or prescribe, and nothing here is a substitute for care from someone who knows your history. The body heals. We write about what supports that design.
Questions people ask
- Is more red light better?
- No, and this is the defining feature of the field. Photobiomodulation shows a biphasic dose response: low levels stimulate and repair tissue better than higher levels do. Past the peak, the benefit falls away again.
- What is the biphasic dose response?
- A curve that rises to a peak and then declines, often called the Arndt-Schulz curve. In photobiomodulation it has been shown in cell studies and in animal experiments, and it applies to ATP production, mitochondrial membrane potential and neurological outcomes.
- Why do red light studies contradict each other?
- The reviewers say it directly: selection of inappropriate dosimetric parameters led to negative studies. If the dose sits past the peak of the curve, a well-run trial will find nothing, and that is not the same as the therapy doing nothing.
- Does it apply to number of sessions as well as intensity?
- Yes. In transcranial work in mice, a distinct biphasic pattern appeared both when the number of treatments was varied and when the energy density of each treatment was varied. Frequency and intensity each have a peak.
- So what dose should I use?
- Nobody can tell you precisely, and anyone who claims to is overselling. What the literature supports is following the parameters a published trial used rather than buying the most powerful device available, and not assuming that longer or closer is better.
