Why Does Red Light Therapy Work for Some People But Not Others?
It comes down to the temperature of your skin at the start and the wavelength you use. This randomized controlled study of 20 healthy volunteers found that near-infrared light increased microcirculatory flow while red light did not, and that only participants whose baseline skin temperature sat in a middle range responded.
Red light therapy, also known as low-level laser therapy (LLLT) or photobiomodulation (PBM), uses specific wavelengths of red and near-infrared light to promote healing and reduce inflammation.
One of the puzzles in PBM research has been why some individuals respond to light therapy while others seem unaffected. This study used advanced noninvasive monitoring to measure microcirculatory activity before, during, and after PBM treatment to find out what determines who benefits.
What the Research Shows
Twenty healthy non-smoking volunteers, 10 men and 10 women, averaging 30 years old, were randomized to receive either red light (633 nm) or near-infrared light (830 nm) applied to the wrist for five minutes. The researchers used photoplethysmography, laser Doppler flowmetry, and thermal imaging to measure microcirculatory blood volume, blood flow, and skin temperature before, during, and for 20 minutes after treatment.
Near-infrared light produced a 27 percent increase in microcirculatory flow, and that increase grew to 54 percent over the 20-minute follow-up period. Red light did not increase median flow. The response was wavelength-dependent.
Baseline skin temperature separated responders from non-responders. Using a threshold of a 0.5°C rise from baseline, only 10 of the 20 participants qualified as responders, and their starting skin temperature fell between 33 and 37.5°C. The non-responders had either hot hands at or above 37.5°C or cold hands at or below 33°C. In responders, the mean time to a 20 percent increase in microcirculatory blood volume and blood flow was under 2.5 minutes from the start of irradiation.
The authors concluded that PBM induces arteriolar vasodilatation, producing both immediate and long-lasting increases in capillary flow and tissue perfusion in healthy individuals.
Dr. Kumar’s Take
This study answers a question I hear all the time: “Will red light work for me?” Two things matter, and neither is what most people assume. The first is wavelength. Near-infrared at 830 nm moved blood flow in this trial and red at 633 nm did not, so the device you buy is not interchangeable with the one your neighbor bought. The second is the state of your skin when you start. Hands that were already hot and hands that were cold both failed to respond. The middle range is where the vascular system had somewhere to go.
That second point is practical. If you are treating a limb that is cold to the touch, warming it before you start is a reasonable thing to try. I would also add that in responders the flow change began fast, inside a couple of minutes, so a session that produces nothing early is unlikely to be rescued by sitting under the panel longer.
Practical Takeaways
- Near-infrared light at 830 nm increased microcirculatory flow by 27 percent during treatment and 54 percent over the following 20 minutes.
- Red light at 633 nm did not increase median microcirculatory flow in this study. The effect was wavelength-dependent.
- Half the participants responded by thermal imaging. Responders started with skin temperature between 33 and 37.5°C.
- Non-responders had hands that were either hot, at or above 37.5°C, or cold, at or below 33°C.
- In responders, blood volume and flow rose 20 percent in under 2.5 minutes on average.
FAQs
Does the wavelength I choose matter?
In this trial, yes. Near-infrared light at 830 nm increased microcirculatory flow both during treatment and across the 20-minute follow-up. Red light at 633 nm did not raise median flow. If circulation is what you are after, that difference is worth paying attention to when choosing a device.
How can I tell if I’m a “responder” to PBM?
In this study, responders were defined by a skin temperature rise of at least 0.5°C from baseline, measured by thermal imaging, and their starting skin temperature was between 33 and 37.5°C. Participants whose hands started hot or cold did not respond. The change also appeared quickly in responders, with blood volume and flow up 20 percent within about two and a half minutes.
Should I do anything before a session?
If your hands or the treatment area run cold, warming up first is worth trying, since the participants who responded started in a middle temperature range rather than at either extreme. This study tested a five-minute exposure over the wrist in healthy volunteers, so treat that as the setting these findings came from.
Bottom Line
Response to red light therapy depended on two things in this randomized trial: the wavelength used and the skin temperature at the start. Near-infrared light at 830 nm raised microcirculatory flow by 27 percent during treatment and 54 percent over the next 20 minutes, while red light at 633 nm did not raise median flow. Only half the participants responded, and they were the ones whose skin started between 33 and 37.5°C. The authors described this as arteriolar vasodilatation producing immediate and lasting gains in capillary flow and tissue perfusion, with direct relevance to tailoring therapy to the individual patient.
