Red Light Therapy - Wavelengths & dose
Can you overdo it? The biphasic dose response
Last updated 30 September 2026. How we write this .
Yes, at least in the lab. Cell and animal studies often find that a moderate amount of red or near-infrared light does more than a larger one, but how far this holds in people is still unclear.
Start here Yes, at least in the lab. Cell and animal studies often find that a moderate amount of red or near-infrared light does more than a larger one, but how far this holds in people is still unclear.
By the end, you will understand Where does the idea of a biphasic dose come from? What did the cell and animal studies actually find? Is the dose just the total amount of light? Key words used in this lesson Placebo An inactive comparison given so that the effect of expectation can be separated from the effect of the intervention itself. Without one, improvement over time cannot be attributed to the treatment. Photobiomodulation The technical term for red light therapy: the use of specific red and near-infrared wavelengths to influence cellular activity. The leading proposed mechanism involves light absorption in the mitochondria. Near-infrared Light just beyond the visible red part of the spectrum, commonly around 800 to 850 nanometres. It penetrates tissue further than visible red light, which is why devices often combine the two. Hormesis The idea that a mild, controlled stress can prompt an adaptive response that leaves the system more resilient. It is the framework usually invoked to explain why heat, cold and exercise might be beneficial - and it implies that more is not automatically better. Full plain-English glossary
Where this information comes from Claim-cited evidence. Central research claims on this lesson are linked to specific studies or official guidance.
This lesson links the specific studies and guidance used for its central claims.
Our standard: expert or athlete opinion may illustrate a viewpoint, but it is never treated as proof. Read the editorial policy .
Report an error or outdated claim .
Yes, in laboratory studies more light can do less. Researchers working on red and near-infrared light, often called photobiomodulation or low-level light therapy, keep finding that a moderate dose changes cells more than a large one, and that a very large dose can cancel or reverse the effect. This is called a biphasic dose response, because the effect rises in one phase and falls in the next. It is well documented in cells and animals. How closely it applies to a person sitting in front of a lamp is much less certain.
Where does the idea of a biphasic dose come from?
The idea is borrowed from pharmacology and toxicology. In the late 19th century two German scientists, Arndt and Schulz, proposed a rule for living things: weak stimuli speed up activity a little, stronger stimuli speed it up more, but past a peak even stronger stimuli suppress it. This became known as the Arndt-Schulz law. Today the broader idea, that a small dose of something can stimulate while a large dose inhibits, is usually called hormesis.
In 2009 Ying-Ying Huang, Michael Hamblin and colleagues at the Wellman Center for Photomedicine in Boston reviewed how this applies to light. Their summary was plain: low levels of light often worked better at stimulating and repairing tissue than higher levels. They also argued that poorly chosen doses help explain why the field has produced a mix of positive and negative trials, and why it is still controversial in mainstream medicine.
What did the cell and animal studies actually find?
One of the clearest examples in the review is a 2003 study by Zhang and colleagues in Hong Kong. They shone red light on human fibroblasts, the cells that make collagen in skin, at a range of doses. According to the review, cell numbers rose by up to 30% at a low dose, while a dose roughly ten times higher actually reduced cell numbers. The original paper describes the response as a curve and picks the peak dose for its later gene experiments.
The pattern is not always a single neat hill. In 1997 Karu and colleagues at the Russian Academy of Sciences exposed four cell models, three from mice and one a human cell line, to pulsed near-infrared laser light across a very wide range of doses. They found not one peak but seven. The peaks fell at the same doses in all four models, which suggests the shape was a real property of the light response rather than a quirk of one cell type.
Repetition matters as well as the size of each dose. In a 2013 mouse study from the Boston group, Xuan and colleagues gave mice with a severe head injury near-infrared light through the skull, either once, on three days in a row, or daily for 14 days. One treatment improved scores for movement and reflexes. Three treatments improved them more. Fourteen gave no benefit over a sham treatment. The mice treated once or three times also had smaller brain lesions after four weeks.
Is the dose just the total amount of light?
Not quite, and this is one of the stranger findings. Light dose is usually reported as energy, which is power multiplied by time. If cells only cared about total energy, a bright light for a short time and a dim light for a long time would have the same effect. Physicists call that assumption reciprocity. Huang's review lists several cell and animal studies where it failed: the same energy delivered at different intensities gave different results.
The team's 2011 update added more detail from inside the cell. Mitochondria are the small structures that make most of a cell's energy. In cell studies, their output of ATP, the molecule cells use as fuel, and the electrical charge across their membranes both followed the rise-then-fall pattern. Reactive oxygen species, chemically active by-products of mitochondria, behaved differently again, with two separate peaks.
Why would more light do less?
Nobody knows for certain, but the reviews offer three explanations. The main target for the light is thought to be cytochrome c oxidase, an enzyme in mitochondria. Light may knock nitric oxide off this enzyme so it works faster, and it raises production of reactive oxygen species. Both molecules cut both ways. At low levels, reactive oxygen species act as signals that switch on repair and growth genes. At high levels they damage cells, and hydrogen peroxide is used in labs precisely to kill cells. Nitric oxide likewise protects or harms depending on how much there is and where. The third idea is that high doses switch on a separate pathway that pushes cells towards programmed death, called apoptosis, which outweighs any benefit.
The state of the tissue also moves the curve. Hamblin's 2018 review of the mechanisms notes that light can have apparently opposite effects in healthy and stressed cells. So the peak found for one tissue, one condition and one measured outcome says little about another.
Does this happen in people?
Human evidence is thinner, but there are hints. In a 2014 Brazilian trial led by Antonialli, 40 untrained men had their thigh muscles treated with a combined laser and LED device at one of three doses, or a placebo, before a bout of hard exercise. The two lower doses improved maximum strength over the following four days, with the middle dose working best. The highest dose did not improve strength, although it still reduced muscle soreness. A dose that is too high for one outcome may still work for another.
Reviews of clinical trials point the same way. In 2010 Steve Tumilty and colleagues found 25 controlled trials of light therapy for tendon problems. Twelve were positive and 13 were inconclusive or showed no effect. The positive trials tended to use doses close to published guidelines, which the authors took as evidence of an effective window. Huang's review adds that the negative trials that reported their light intensity had all gone above those guidelines.
How sure can anyone be?
Less sure than device marketing suggests. Most of the dose-response evidence comes from cells in dishes and from rodents. In people, blood, skin pigment and tissue absorb and scatter light, so only an uncertain fraction of what leaves a device reaches a deeper target. Much of the research comes from a small number of groups, and the 2009 review was co-written by a researcher from a UK laser company, THOR Photomedicine. The 2011 update itself says more work is needed to learn how far biphasic responses apply in treatment. The idea that more is not always better is well supported. Where the peak sits for a particular person, body part and goal is not known, which is why this lesson gives no settings.
This lesson is for education only and is not medical advice. If you have pain, an injury or a health condition, speak to your GP or a qualified health professional.
Related in this classroom The studies behind this page Biphasic dose response in low level light therapy - Dose-Response, 2009Biphasic dose response in low level light therapy - an update - Dose-Response, 2011Mechanisms and mitochondrial redox signaling in photobiomodulation - Photochemistry and Photobiology, 2018cDNA microarray analysis of gene expression profiles in human fibroblast cells irradiated with red light - Journal of Investigative Dermatology, 2003Nonmonotonic behavior of the dose dependence of the radiation effect on cells in vitro exposed to pulsed laser radiation at lambda = 820 nm - Lasers in Surgery and Medicine, 1997Transcranial low-level laser therapy improves neurological performance in traumatic brain injury in mice: effect of treatment repetition regimen - PLoS One, 2013Phototherapy in skeletal muscle performance and recovery after exercise: effect of combination of super-pulsed laser and light-emitting diodes - Lasers in Medical Science, 2014Low level laser treatment of tendinopathy: a systematic review with meta-analysis - Photomedicine and Laser Surgery, 2010Sources for this classroom
Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.