Red light therapy uses either red light, near-infrared light, or a combination of both. But what exactly is the difference between red light and near-infrared light?

All light travels as a wave of electromagnetic radiation, and the distance between wave peaks is known as the wavelength. The color and type of light are characterized by its wavelength.

Red light has wavelengths in the range of about 620–700 nm, while near-infrared light has wavelengths in the range of about 700–1,400 nm. The defining difference between the two types of light is that red light is visible to the human eye and infrared light is not.

Our bodies are mostly impenetrable to visible light, including red light. However, longer-wavelength light (i.e., closer to near-infrared than red) can more readily penetrate deeper into the body.^{[24][25][26][27]} As a result, red light therapy for topical applications (e.g., hair loss, skin health, eyes) will tend to use red light (since a greater proportion of energy is delivered to skin cells), while red light therapy targeting tissues inside the body (e.g., joints, muscles, brain) will tend to use infrared light.

Much of the light emitted by the sun is in the red and near-infrared spectrum. As a result, sunlight exposure might be able to replicate some of the effects of red light therapy.

It’s estimated that unobscured sunlight delivers around 20–40 mW/cm² of red and near-infrared light (although the amount varies depending on factors like time of day, time of year, and latitude).^[28] This puts sunlight in the range of the power densities used in many red light therapy studies.

That said, there isn’t much direct research looking at whether sunlight actually does replicate the health effects of red light therapy, so whether sun exposure can act as a form of red light therapy remains uncertain.

Furthermore, sunlight is a major source of other wavelengths of light as well, which could enhance or inhibit the effects of red light. For example, while exposure to red and infrared light may reduce signs of skin aging (although the evidence for this is weak), the net effect of getting a lot of sunlight exposure is the promotion of skin aging due to the effect of ultraviolet light.^[29][30] With that said, it may be worth noting that most sunscreens block UV light while not blocking much red and infrared light (although a small number of sunscreens block both).^[31][32][33]

Ultimately, whether the net effect of sunlight is similar to that of red and near-infrared light can’t be assumed.

Due to its suspected ability to increase cellular energy production, red light therapy has been investigated as an ergogenic aid, with studies typically exposing the working muscle to light immediately before exercise.

Red light therapy has been shown to slightly increase the number of repetition to failure on eccentric exercises (e.g., bicep curls and knee extensions), although this finding is based in large part on studies involving young male volleyball players performed by a single research group.^[34]

Only a couple of long-term studies have looked at whether red light therapy can increase strength and muscle gains when added to regular resistance training. One study in young men found red light therapy before strength training led to greater increases in muscle size and strength,^[35] whereas a study in older men found it had no benefit.^[36] Finally, one study on older women found red light therapy after strength training did not increase muscle strength (muscle size was not reported).^[37]

Red light therapy before exercise has sometimes been found to increase performance on predominantly anaerobic activities (e.g., cycling or running to exhaustion)^{[38][39][40][41]} However, this is not a consistent finding.^[42][43][44]

Lastly, red light therapy does not seem to reliably reduce muscle soreness in the days following a workout.^{[45][46][47][47]}

Red light therapy is sometimes used as a treatment for acne, possibly by reducing inflammation and sebum production.^[48] However, most of the research on this topic is not very high-quality (often lacking appropriate control groups, for example).^{[49][50][51][52][53]}

Some research suggests red light therapy can increase the dermal synthesis of collagen and elastin,^[54][55] proteins that help maintain skin elasticity and which tend to decrease during aging. Consistent with this, a few clinical trials suggest red light therapy can indeed reduce the appearance of wrinkles,^[56] although only a few studies exist, so more research is needed.

Finally, it’s been suggested that red light exposure may make the skin less susceptible to sunburns. The evidence for this draws largely from a single pilot study involving 15 people. In that study, regularly exposing the skin to red light — specifically a wavelength of 660 nm at an intensity of 60 mW/cm² for 160 seconds — seemed to result in less erythema (redness) in response to UV exposure (although statistical significance was not examined).^[57] Subsequent (unpublished) data from the same researcher indicated that this protective effect was greater with near-infrared light than red light. Ultimately, given the preliminary nature of this data, this finding requires replication. And even if red/near-infrared light does in fact have a sunburn-inhibiting effect, it can’t yet be assumed this translates into a reduction in UV-induced DNA damage, meaning that whether red light therapy reduces the risk of skin cancer is unclear.