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Light therapy
Light therapy
Light therapy
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Light therapy
Example of light therapy for winter depression
ICD-10-PCS6A6, GZJ
ICD-999.83, 99.88
MeSHD010789

Light therapy, also called phototherapy or bright light therapy, is the exposure to direct sunlight or artificial light at controlled wavelengths in order to treat a variety of medical disorders, including seasonal affective disorder (SAD), circadian rhythm sleep-wake disorders, cancers, neonatal jaundice, and skin wound infections. Treating skin conditions such as neurodermatitis, psoriasis, acne vulgaris, and eczema with ultraviolet light is called ultraviolet light therapy.

Medical uses

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A baby in neonatal care under a blue-light (420–470 nm) phototherapy lamp wearing only a diaper, being treated for newborn jaundice (hyperbilirubinemia)

Nutrient deficiency

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Vitamin D deficiency

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Exposure to UV-B light at wavelengths of 290-300 nanometers enables the body to produce vitamin D3 to treat vitamin D3 deficiency.[1]

Skin conditions

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High-intensity blue light (425 nm) used for the attempted treatment of acne

Light therapy treatments for the skin usually involve exposure to ultraviolet light.[2] The exposures can be to a small area of the skin or over the whole body surface, as in a tanning bed. The most common treatment is with narrowband UVB, which has a wavelength of approximately 311–313 nanometers. Full body phototherapy can be delivered at a doctor's office or at home using a large high-power UVB booth.[3] Tanning beds, however, generate mostly UVA light, and only 4% to 10% of tanning-bed light is in the UVB spectrum.

Acne vulgaris

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As of 2012, evidence for light therapy and lasers in the treatment of acne vulgaris was not sufficient to recommend them.[4] There is moderate evidence for the efficacy of blue and blue-red light therapies in treating mild acne, but most studies are of low quality.[5][6] While light therapy appears to provide short-term benefit, there is a lack of long-term outcome data in those with severe acne.[7]

Atopic dermatitis

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Light therapy is considered one of the best monotherapy treatments for atopic dermatitis (AD) when applied to patients who have not responded to traditional topical treatments. The therapy offers a wide range of options: UVA1 for acute AD, NB-UVB for chronic AD, and balneophototherapy have proven their efficacy. Patients tolerate the therapy safely but, as in any therapy, there are potential adverse effects and care must be taken in its application, particularly to children.[8] According to a study involving 21 adults with severe atopic dermatitis, narrowband UVB phototherapy administered three times per week for 12 weeks reduced atopic dermatitis severity scores by 68%. In this open study, 15 patients still experienced long-term benefits six months later.[9]

Cancer

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According to the American Cancer Society, there is some evidence that ultraviolet light therapy may be effective in helping treat certain kinds of skin cancer, and ultraviolet blood irradiation therapy is established for this application. However, alternative uses of light for cancer treatment – light box therapy and colored light therapy – are not supported by evidence.[10] Photodynamic therapy (often with red light) is used to treat certain superficial non-melanoma skin cancers.[11]

Psoriasis

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See also: Goeckerman therapy

For psoriasis, UVB phototherapy has been shown to be effective.[12] A feature of psoriasis is localized inflammation mediated by the immune system.[13] Ultraviolet radiation is known to suppress the immune system and reduce inflammatory responses. Light therapy for skin conditions like psoriasis usually use 313 nanometer UVB though it may use UVA (315–400 nm wavelength) or a broader spectrum UVB (280–315 nm wavelength). UVA combined with psoralen, a drug taken orally, is known as PUVA treatment. In UVB phototherapy the exposure time is very short, seconds to minutes depending on intensity of lamps and the person's skin pigment and sensitivity.

Vitiligo

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About 1% of the human population has vitiligo which causes painless distinct light-colored patches of the skin on the face, hands, and legs. Phototherapy is an effective treatment because it forces skin cells to manufacture melanin to protect the body from UV damage. Prescribed treatment is generally 3 times a week in a clinic or daily at home. About 1 month usually results in re-pigmentation in the face and neck, and 2–4 months in the hands and legs. Narrowband UVB is more suitable to the face and neck and PUVA is more effective at the hands and legs.[14]

Other skin conditions

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Some types of phototherapy may be effective in the treatment of polymorphous light eruption, cutaneous T-cell lymphoma[15] and lichen planus. Narrowband UVB between 311 and 313 nanometers is the most common treatment.[16]

Retinal conditions

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There is preliminary evidence that light therapy is an effective treatment for diabetic retinopathy and diabetic macular oedema.[17][18]

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Seasonal affective disorder

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Main article: Seasonal affective disorder

The effectiveness of light therapy for treating seasonal affective disorder (SAD) may be linked to reduced sunlight exposure in the winter months. Light resets the body's internal clock.[19] Studies show that light therapy helps reduce the debilitating depressive symptoms of SAD, such as excessive sleepiness and fatigue, with results lasting for at least 1 month. Light therapy is preferred over antidepressants in the treatment of SAD because it is a relatively safe and easy therapy with minimal side effects.[20] Two methods of light therapy, bright light and dawn simulation, have similar success rates in the treatment of SAD.[21]

It is possible that response to light therapy for SAD could be season dependent.[22] Morning therapy has provided the best results because light in the early morning aids in regulating the circadian rhythm.[20] People affected by SAD often have low energy, tend to eat more carbohydrates and sleep longer, but symptoms can vary between people.[23]

A Cochrane review conducted in 2019 states the evidence that light therapy's effectiveness as a treatment for the prevention of seasonal affective disorder is limited, although the risk of adverse effects are minimal. Therefore, the decision to use light therapy should be based on a person's preference of treatment.[24]

Non-seasonal depression

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Light therapy has also been suggested in the treatment of non-seasonal depression and other psychiatric mood disturbances, including major depressive disorder,[25][26] bipolar disorder and postpartum depression.[27][28] A meta-analysis by the Cochrane Collaboration concluded that "for patients suffering from non-seasonal depression, light therapy offers modest though promising antidepressive efficacy."[29] A 2008 systematic review concluded that "overall, bright light therapy is an excellent candidate for inclusion into the therapeutic inventory available for the treatment of nonseasonal depression today, as adjuvant therapy to antidepressant medication, or eventually as stand-alone treatment for specific subgroups of depressed patients."[30] A 2015 review found that supporting evidence for light therapy was limited due to serious methodological flaws.[31]

A 2016 meta-analysis showed that bright light therapy appeared to be efficacious, particularly when administered for 2–5 weeks' duration and as monotherapy.[32]

Chronic circadian rhythm sleep disorders (CRSD)

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In the management of circadian rhythm disorders such as delayed sleep phase disorder (DSPD), the timing of light exposure is critical. Light exposure administered to the eyes before or after the nadir of the core body temperature rhythm can affect the phase response curve.[33] Use upon awakening may also be effective for non-24-hour sleep–wake disorder.[34] Some users have reported success with lights that turn on shortly before awakening (dawn simulation). Evening use is recommended for people with advanced sleep phase disorder. Some, but not all, totally blind people whose retinae are intact, may benefit from light therapy.

Circadian rhythm sleep disorders and jet lag

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Main article: Circadian rhythm sleep disorder

Source:[35]

Situational CRSD
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Light therapy has been tested for individuals with shift work sleep disorder and for jet lag.[36][37]

Sleep disorder in Parkinson's disease
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Light therapy has been trialed in treating sleep disorders experienced by patients with Parkinson's disease.[38]

Sleep disorder in Alzheimer's disease
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Studies have shown that daytime and evening light therapy for nursing home patients with Alzheimer's disease, who often struggle with agitation and fragmented wake/rest cycles effectively led to more consolidated sleep and an increase in circadian rhythm stability.[39][40][41]

Neonatal jaundice (postnatal jaundice)

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Further information: Neonatal jaundice and Bili light
A newborn infant undergoing white-light phototherapy to treat neonatal jaundice

Light therapy is used to treat cases of neonatal jaundice.[42] Bilirubin, a yellow pigment normally formed in the liver during the breakdown of old red blood cells, cannot always be effectively cleared by a neonate's liver causing neonatal jaundice. Accumulation of excess bilirubin can cause central nervous system damage, and so this buildup of bilirubin must be treated. Phototherapy uses the energy from light to isomerize the bilirubin and consequently transform it into compounds that the newborn can excrete via urine and stools. Bilirubin is most successful absorbing light in the blue region of the visible light spectrum, which falls between 460 and 490 nm.[43] Therefore, light therapy technologies that utilize these blue wavelengths are the most successful at isomerizing bilirubin.[44]

Techniques

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Photodynamic therapy

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Main article: Photodynamic therapy

Photodynamic therapy (PDT) is a form of phototherapy using nontoxic light-sensitive compounds (photosensitizers) that are exposed selectively to light at a controlled wavelength, laser intensity, and irradiation time, whereupon they generate toxic reactive oxygen species (ROS) that target malignant and other diseased cells. Oxygen is thus required for activity, lowering efficacy in highly developed tumors and other hypoxic environments. Selective apoptosis of diseased cells is difficult due to the radical nature of ROS, but may be controlled for through membrane potential and other cell-type specific properties'[45] effects on permeability or through photoimmunotherapy. In developing any phototherapeutic agent, the phototoxicity of the treatment wavelength should be considered.

Photodynamic cancer therapy

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Various cancer treatments utilizing PDT have been approved by the FDA. Treatments are available for actinic keratosis (blue light with aminolevulinic acid), cutaneous T-cell lymphoma, Barrett esophagus, basal cell skin cancer, esophageal cancer, non-small cell lung cancer, and squamous cell skin cancer (Stage 0). Photosensitizing agents clinically-approved or undergoing clinical trials for the treatment of cancers include Photofrin, Temoporfin, Motexafin lutetium, Palladium bacteriopheophorbide, Purlytin, and Talaporfin. Verteporfin is approved to treat eye conditions such as macular degeneration, myopia, and ocular histoplasmosis.[46] Third-generation photosensitizers are currently in development, but none are yet approved for clinical trials.

Antimicrobial photodynamic therapy

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PDT may also be utilized to treat multidrug-resistant skin, wound, or other superficial infections. This is known as antimicrobial photodynamic therapy (aPDT) or photodynamic inactivation (PDI). aPDT has been observed to be effective against both gram-positive and gram-negative bacteria such as Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Mycobacterium. aPDT has shown lowered efficacy on some other bacterial species, such as Klebsiella pneumoniae and Acinetobacter baumannii. This is likely due to factors such as cell wall thickness and membrane potential.[45] Many studies utilizing aPDT focus on the application of the photosensitizer through leakage from a hydrogel, which has been found to increase wound healing speed of skin infections[47][48] through the upregulation of vascular endothelial growth factor (VEGF) and hypoxia inducible factor (HIF).[49] This controlled leakage allows for prolonged but limited generation of ROS, lowering the impact on human cell viability due to ROS cytotoxicity. It is unlikely for drug resistance to photosensitizers to form due to the nontoxic nature of the photosensitizer itself as well as the ROS generation mechanism of action, which cannot be prevented outside of hypoxic environments. Certain dental infections (peri-implantitis, periodontitis) are more difficult to treat with PDT as opposed to photothermal therapy due to the requirement of oxygen, though a significant response is still observed.[50][51][52]

Increased antimicrobial activity and wound healing speeds are typically observed when PDT is combined with photothermal therapy in photodynamic/photothermal combination therapy.

Photothermal Therapy

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Main article: Photothermal therapy

Photothermal therapy (PTT) is a form of phototherapy that uses non-toxic compounds called photothermal agents (PTA) that, when irradiated at a certain wavelength of light, converts the light energy directly to heat energy. The photothermal conversion efficiency determines the amount of light converted to heat, which can dictate the necessary irradiation time and/or laser intensity for treatments. Typically PTT treatments use wavelengths in the near-infrared (NIR) spectra, which can be further divided into NIR-I (760-900 nm), NIR-II (900-1880 nm), and NIR-III (2080-2340 nm) windows.[53] Wavelengths in these regions are typically less phototoxic than UV or high-energy visible light. In addition, NIR-II wavelengths have been observed to show deeper penetration than NIR-I wavelengths, allowing for treatment of deeper wounds, infections, and cancers. Important considerations for the development of a PTA include photothermal conversion efficiency, phototoxicity, laser intensity, irradiation time, and the temperature at which human cell viability is impaired (around 46-60 °C).[54] Currently, the only FDA-approved photothermal agent is indocyanine green which is active against both tumor and bacterial cells.[50][55]

PTT is less selective than photodynamic therapy (PDT, see above) due to its heat-based mechanism of action, but also less likely to promote drug resistance than most, if not all, currently developed treatments. In addition, PTT can be used in hypoxic environments and on deeper wounds, infections, and tumors than PDT due to the higher wavelength of light. Due to PTT activity in hypoxic environments, it may be also used on more developed tumors than PDT. Low-temperature PTT (≤ 45 °C) for treatment of infections is also a possibility when combined with an antibiotic compound due to heat's proportionality with membrane permeability - a hotter environment causes heightened membrane permeability, which thus allows the drug into the cell.[56] This would reduce/eliminate the impact on human cell viability, and aiding in antibiotic accumulation within the target cell may assist in restoring activity in antibiotics that pathogens had developed resistance to.

PTT is typically seen to have improved antimicrobial and wound healing activity when combined with an additional mechanism of action through PDT or added antibiotic compounds in the application.

See also: Combined photothermal and photodynamic therapy

Light boxes

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Light intensity of a light therapy lamp in a room. Daylight only penetrates into the room filtered and restricted from the window curtain and protruding roof. In modern society, people often spend too little time outdoors, where the light is significantly brighter than in closed rooms.
"Light box" redirects here. For other uses, see Lightbox (disambiguation).

The production of the hormone melatonin, a sleep regulator, is inhibited by light and permitted by darkness as registered by photosensitive ganglion cells in the retina.[57] To some degree, the reverse is true for serotonin,[58] which has been linked to mood disorders. Hence, for the purpose of manipulating melatonin levels or timing, light boxes providing very specific types of artificial illumination to the retina of the eye are effective.[59]

Light therapy uses either a light box which emits up to 10,000 lux of light at a specified distance,[a] much brighter than a customary lamp, or a lower intensity of specific wavelengths of light from the blue (460 nm) to the green (525 nm) areas of the visible spectrum.[60] A 1995 study showed that green light therapy at doses of 350 lux produces melatonin suppression and phase shifts equivalent to 10,000 lux white light therapy,[61][62] but another study published in May 2010 suggests that the blue light often used for SAD treatment should perhaps be replaced by green or white illumination, because of a possible involvement of the cones in melatonin suppression.[63]

Risks and complications

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Ultraviolet

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Ultraviolet light causes progressive damage to human skin and erythema even from small doses.[64][65] This is mediated by genetic damage, collagen damage, as well as destruction of vitamin A and vitamin C in the skin and free radical generation.[citation needed] Ultraviolet light is also known to be a factor in formation of cataracts.[66][67] Ultraviolet radiation exposure is strongly linked to incidence of skin cancer.[68][64][69]

Visible light

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Optical radiation of any kind with enough intensity can cause damage to the eyes and skin including photoconjunctivitis and photokeratitis.[70] Researchers have questioned whether limiting blue light exposure could reduce the risk of age-related macular degeneration.[71] According to the American Academy of Ophthalmology, there is no scientific evidence showing that exposure to blue light emitting devices result in eye damage.[72] According to Harriet Hall, blue light exposure is reported to suppress the production of melatonin, which affects our body's circadian rhythm and can decrease sleep quality.[73] It is reported that, in reproductive-age females, bright light therapy may activate the production of reproductive hormones, such as luteinizing hormone, follicle-stimulating hormone, and estradiol[74]

Modern phototherapy lamps used in the treatment of seasonal affective disorder and sleep disorders either filter out or do not emit ultraviolet light and are considered safe and effective for the intended purpose, as long as photosensitizing drugs are not being taken at the same time and in the absence of any existing eye conditions. Light therapy is a mood altering treatment, and just as with drug treatments, there is a possibility of triggering a manic state from a depressive state, causing anxiety and other side effects. While these side effects are usually controllable, it is recommended that patients undertake light therapy under the supervision of an experienced clinician, rather than attempting to self-medicate.[75]

Contraindications to light therapy for seasonal affective disorder include conditions that might render the eyes more vulnerable to phototoxicity, tendency toward mania, photosensitive skin conditions, or use of a photosensitizing herb (such as St. John's wort) or medication.[76][77] Patients with porphyria should avoid most forms of light therapy. Patients on certain drugs such as methotrexate or chloroquine should use caution with light therapy as there is a chance that these drugs could cause porphyria.[citation needed]

Side effects of light therapy for sleep phase disorders include jumpiness or jitteriness, headache, eye irritation and nausea.[78] Some non-depressive physical complaints, such as poor vision and skin rash or irritation, may improve with light therapy.[79]

History

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Child patients with external forms of tuberculosis, especially of the bones and joints, laying on beds on a terrace outside Treloar Hospital in Alton, Hampshire, England, in sunlight as part of their light therapy, ca. first half of the 20th century[80]

Many ancient cultures practiced various forms of heliotherapy, including people of Ancient Greece, Ancient Egypt, and Ancient Rome.[81] The Inca, Assyrian and early Germanic peoples also worshipped the sun as a health bringing deity. Indian medical literature dating to 1500 BCE describes a treatment combining herbs with natural sunlight to treat non-pigmented skin areas. Buddhist literature from about 200 CE and 10th-century Chinese documents make similar references.

The Faroese physician Niels Finsen is believed to be the father of modern phototherapy. He developed the first artificial light source for this purpose.[82] Finsen used short wavelength light to treat lupus vulgaris, a skin infection caused by Mycobacterium tuberculosis. He thought that the beneficial effect was due to ultraviolet light killing the bacteria, but recent studies showed that his lens and filter system did not allow such short wavelengths to pass through, leading instead to the conclusion that light of approximately 400 nanometers generated reactive oxygen that would kill the bacteria.[83] Finsen also used red light to treat smallpox lesions. He received the Nobel Prize in Physiology or Medicine in 1903.[84] Scientific evidence for some of his treatments is lacking, and later eradication of smallpox and development of antibiotics for tuberculosis rendered light therapy obsolete for these diseases.[85] In the early 20th-century light therapy was promoted by Auguste Rollier and John Harvey Kellogg.[86] In 1924, Caleb Saleeby founded The Sunlight League.[87]

From the late nineteenth century until the early 1930s, light therapy was considered an effective and mainstream medical therapy in the UK for conditions such as varicose ulcer, 'sickly children' and a wide range of other conditions. Controlled trials by the medical scientist Dora Colebrook, supported by the Medical Research Council, indicated that light therapy was not effective for such a wide range of conditions.[88]

Controversy

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Red light therapy involves exposure to low levels of red light or near-infrared light, typically through lamps or masks.[89] It is promoted for various skin-related benefits, including improved appearance and reduced signed of aging.[89][90][91] However, there is currently insufficient scientific evidence to support many of these claims.[91] There has been some indication that it may reduce inflammation associated with conditions such as acne or rosacea, but evidence supporting its anti-aging effects remain limited.[89] Most existing research has focused on in-office treatments, while at-home devices are generally less powerful and precise, which may lead to inconsistent results.[89][90] It is generally considered safe, however if misused red light therapy could cause eye or skin damage.[92][89][91]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Light therapy

View on Grokipedia
from Grokipedia
Light therapy, also known as phototherapy, is a non-invasive treatment that involves controlled exposure to specific wavelengths and intensities of light—either from natural or artificial sources such as light boxes, lamps, or lasers—to alleviate symptoms of various conditions, including mood disorders, disturbances, and diseases. The therapy's mechanisms vary by type but generally influence biological processes like regulation, levels, cellular , and inflammation reduction, making it a versatile option for both psychiatric and dermatological applications. One of the primary forms, bright light therapy (BLT), uses high-intensity white or full-spectrum light (typically 10,000 lux for 20–30 minutes daily) to mimic natural sunlight and treat , a subtype of characterized by recurrent winter-onset symptoms such as low mood, fatigue, and carbohydrate cravings due to shortened daylight hours. BLT is thought to work by suppressing production and boosting serotonin activity in the brain via the , thereby correcting phase delays in circadian rhythms. Clinical evidence supports its efficacy, with remission rates of up to 61% in SAD patients compared to 32% with , and benefits extending to non-seasonal depression (effect size 0.53), bipolar depression, and even adjunctive use in eating disorders like . Sessions are most effective when administered in the morning shortly after waking, and devices must filter (UV) rays to prevent eye damage. In , ultraviolet phototherapy employs UVB (medium-wavelength, targeting superficial skin layers) or UVA (longer-wavelength, penetrating deeper) light, often combined with photosensitizing agents like (PUVA), to manage chronic inflammatory skin conditions such as , eczema (), and . This approach slows excessive skin cell proliferation, reduces inflammation, and promotes repigmentation by stimulating melanocytes, with treatment courses typically involving 2–3 sessions per week for 6–12 weeks. It is also used for neonatal jaundice via blue light (bililights) to break down and for , where light activates a topical to destroy precancerous or cancerous skin cells. Evidence from clinical guidelines indicates significant symptom improvement in 70–80% of psoriasis patients, though long-term use requires monitoring for skin aging or cancer risk. Low-level light therapy (LLLT), or photobiomodulation, utilizes non-thermal red (600–700 nm) and near-infrared (700–1,100 nm) light from lasers or light-emitting diodes (LEDs) to stimulate mitochondrial function, increase ATP production, and modulate for tissue repair; effective devices use engineered parameters with specific narrow wavelengths, sufficient irradiance (50–200 mW/cm²), and focused beams for tissue penetration, distinguishing them from ordinary red lights that provide minimal to no therapeutic benefit. Red light therapy, a form of LLLT, employs red light (630–660 nm) and near-infrared light (800–850 nm) to penetrate tissues, reduce inflammation, improve blood flow, and stimulate cellular repair. Applications include (reducing wrinkles and improving texture in 90% of treated s by increasing collagen production and skin elasticity), treatment (via blue-red combinations killing Propionibacterium acnes ), and healing (accelerating recovery by 50%), and reduction in hypertrophic keloids, as well as treatment of skin redness (e.g., in rosacea) and psoriasis. It also shows promise for oral prevention in cancer s and regrowth in androgenetic alopecia, reduction of joint pain and muscle soreness in conditions like arthritis, and preliminary evidence for alleviating dementia symptoms. Studies report high satisfaction with minimal downtime, though optimal (e.g., 4–50 J/cm²) remains key to efficacy; while evidence supports these uses, results vary and more large-scale studies are needed, with limited support for applications such as weight loss, cellulite reduction, cancer treatment, or broad mental health benefits. Overall, light therapy is well-tolerated across modalities, with common side effects limited to mild , , , or temporary redness, but contraindications include disorders, certain eye conditions (e.g., ), and active . Its accessibility—via home devices or clinical settings—has made it a first-line or adjunctive option, supported by decades of research demonstrating both rapid onset (within days for ) and sustained benefits when maintained seasonally or as needed.

Principles and Mechanisms

Definition and Classification

Light therapy, also known as phototherapy, refers to the controlled exposure of the body to specific wavelengths of light for therapeutic purposes, aimed at treating various medical conditions through non-invasive means. This distinguishes it from general environmental illumination, which lacks targeted therapeutic intent, or tanning practices, which primarily involve uncontrolled (UV) exposure for cosmetic effects. The therapy leverages light's interaction with biological tissues to induce physiological responses, such as regulation or cellular repair, without relying on pharmacological agents. The term "phototherapy" has evolved from earlier concepts like "heliotherapy," which denoted treatment using natural and dates back to ancient civilizations, including Greek and Egyptian practices for skin ailments. By the 19th and early 20th centuries, heliotherapy gained prominence in European sanatoriums for and , as documented in from that era. Modern phototherapy emerged in the mid-20th century with the advent of artificial light sources, shifting the focus to precise, indoor applications and broadening the terminology to encompass diverse light spectra beyond . Light therapy is broadly classified into several types based on the and delivery mechanism. Visible therapy typically employs bright white or colored in the 400-700 nm range, often used for mood and disorders. therapy for dermatological treatments like primarily uses UVA (315-400 nm, often with ) and UVB (280-315 nm). UVC (100-280 nm) is used cautiously, mainly for sterilization rather than direct therapy, due to its high and potential for damage. therapy utilizes wavelengths above 700 nm (near-infrared: 700-1400 nm; far-infrared: beyond 1400 nm) for pain relief and . Laser-based therapies, involving coherent from lasers, span various wavelengths and are applied in targeted procedures like for tissue regeneration. Key parameters in light therapy include wavelength, which determines the light's penetration and biological effect; intensity, measured in lux for visible light (e.g., 2,500-10,000 lux for bright light therapy) or milliwatts per square centimeter (mW/cm²) for other spectra; duration of exposure, ranging from minutes to hours per session; and delivery methods such as fluorescent lamps, light-emitting diodes (LEDs), or lasers, which ensure uniform and safe application. These parameters are tailored to the therapeutic goal, with safety guidelines established by organizations like the American Academy of Dermatology to minimize risks such as skin irritation.

Biological Mechanisms

Light therapy exerts its effects through phototransduction, the process by which is absorbed by biological chromophores, initiating photochemical reactions that influence cellular and systemic functions. In the , opsins—light-sensitive G-protein-coupled receptors—absorb photons, triggering a cascade of signaling events that convey light to the . Similarly, in the skin, chromophores such as and opsins detect light, leading to responses like melanin synthesis and calcium mobilization in melanocytes. Key biological pathways activated by light include circadian entrainment mediated by the (SCN), the master circadian pacemaker in the . Light signals from intrinsically photosensitive ganglion cells (ipRGCs) project to the SCN via the , synchronizing molecular clocks to the environmental light-dark cycle and influencing sleep-wake rhythms and mood regulation. Additionally, light suppresses production in the while stimulating serotonin synthesis in the , contributing to effects through enhanced serotonergic activity. At the cellular level, low-level light therapy (LLLT) stimulates mitochondria by exciting , a key chromophore in the , which elevates mitochondrial and boosts ATP production through enhanced . In ultraviolet (UV) phototherapy, UVB radiation induces DNA damage such as cyclobutane in , prompting pathways that modulate immune responses in conditions like . Wavelength specificity is critical: blue light (460-480 nm) potently activates in ipRGCs, driving non-visual effects like pupillary constriction and circadian phase shifting, while UVB (290-315 nm) converts 7-dehydrocholesterol in the skin to previtamin D3, which thermally isomerizes to D3, supporting calcium and immune function. The efficacy of phototherapy often follows dose-response principles governed by the Bunsen-Roscoe reciprocity law, which posits that the biological effect depends on the total energy dose ( multiplied by exposure time) rather than their individual values, provided the dose remains within linear photochemical limits. This reciprocity holds for many light-induced reactions but may deviate at very low or high intensities due to nonlinear cellular responses.

Clinical Applications

Psychiatric and Sleep Disorders

Light therapy has been established as a primary treatment for (SAD), a subtype of characterized by recurrent depressive episodes during fall and winter months, with a prevalence estimated at 5-10% in temperate climates. Clinical trials demonstrate response rates of 50-80% in symptom improvement when using bright light therapy at intensities of 10,000 lux for 30 minutes daily, typically administered in the morning to mimic natural daylight and suppress while boosting serotonin levels. A of nine randomized controlled trials confirmed that this approach leads to significant reductions in depressive symptoms, with effect sizes indicating efficacy comparable to . In non-seasonal depression, including (MDD), light therapy serves as an effective adjunctive treatment, particularly for patients with atypical features or incomplete response to antidepressants. Meta-analyses of randomized trials show moderate effect sizes (around 0.5-0.8) for symptom reduction, on par with standard antidepressants, when light therapy is combined with existing regimens. For instance, morning exposure to 10,000 lux for 30-60 minutes has been associated with improved mood scores on scales like the Hamilton Depression Rating Scale, with benefits emerging within 1-2 weeks. For circadian rhythm sleep disorders (CRSD), such as delayed sleep phase syndrome (DSPS), light therapy targets phase advancement through timed morning exposure to reset the endogenous clock. Systematic reviews indicate that 2-3 hours of bright light (2,500-10,000 ) shortly after desired wake time advances sleep onset by 1-2 hours. This protocol is complemented by evening strategies to prevent phase delays, such as dim lighting, blue-light blocking glasses, and minimizing screen time after 7:00–8:00 PM. In jet lag protocols, pre-flight exposure for 3-5 days—such as morning light for eastward travel—facilitates faster adaptation. Light therapy also shows promise in bipolar disorder for prophylactic use against seasonal relapses, with morning sessions in autumn and winter recommended to stabilize mood without triggering when mood stabilizers are co-administered. Clinical guidelines from expert consensus emphasize daily dosing of 10,000 for 30 minutes, which has been linked to fewer depressive episodes in bipolar patients with seasonal patterns. Entrainment protocols for these applications often rely on dim light melatonin onset (DLMO) testing to personalize timing, ensuring light exposure occurs in the early morning (typically 8-10 hours after DLMO) for optimal phase shifts in both psychiatric and sleep contexts. This biomarker-guided approach, validated in chronotherapy studies, enhances efficacy by aligning treatment with individual circadian phases, as measured via salivary assays.

Dermatological Conditions

Light therapy has proven effective in treating various dermatological conditions by leveraging specific wavelengths to target skin pathology, such as , bacterial overgrowth, and abnormal , through mechanisms like and photodynamic reactions. In , narrowband ultraviolet B (NB-UVB) phototherapy at 311 nm serves as a first-line treatment for moderate-to-severe cases, inducing remission by suppressing proliferation and inflammatory cytokines. Typical protocols involve sessions three times per week for 3 months or more, with patients often requiring 20-36 exposures to achieve significant improvement. Clinical studies report 70-80% of patients attaining at least 75% improvement in the (PASI) score after this regimen, with relapse rates comparable to other therapies when maintenance sessions are used. Red light therapy, typically at 630-660 nm, has also shown benefits in managing psoriasis symptoms, including reducing redness and inflammation, with evidence from clinical trials indicating improvements in plaque severity. For acne vulgaris, blue light therapy at approximately 415 nm targets the porphyrins produced by Propionibacterium acnes, leading to bacterial destruction via photodynamic excitation without significant penetration. Combining blue light with (around 630-660 nm) enhances outcomes by reducing through modulation of cytokines and activity, making it suitable for mild-to-moderate inflammatory lesions. Evidence from randomized trials indicates partial or complete remission in up to 92% of patients with consistent use over 4-8 weeks, though long-term efficacy requires ongoing treatment to prevent recurrence. Red light therapy has demonstrated efficacy in skin rejuvenation by increasing collagen production, reducing wrinkles, improving skin elasticity, and aiding in the treatment of scars, including acne and hypertrophic scars. Clinical studies report significant improvements in skin texture and elasticity, with reductions in wrinkle depth by 20-30% after 8-12 weeks of treatment at 630-660 nm wavelengths. Additionally, for hair growth in androgenetic alopecia, FDA-cleared low-level red light devices promote regrowth, with meta-analyses showing increased hair density by 20-30 hairs per cm² compared to controls after 16-26 weeks of use. While results vary and larger-scale studies are needed, these applications highlight red light therapy's role in non-invasive dermatological care. Narrowband UVB therapy also benefits and eczema by alleviating and through downregulation of pro-inflammatory cytokines like IL-4 and IL-13, as well as inducing in inflammatory cells. Administered 2-3 times weekly, it improves skin barrier function and reduces severity, with studies showing moderate-to-marked clearance in 60-70% of patients after 12-24 sessions. This approach is particularly valuable for widespread or cases, offering sustained relief with minimal systemic effects. Vitiligo responds well to NB-UVB, which stimulates migration and proliferation to promote repigmentation, especially in facial lesions where response rates are higher due to better follicular reservoirs. Treatment typically involves twice-weekly sessions for 6-12 months, yielding 50-75% repigmentation in responsive areas for over half of patients. Long-term maintenance may be needed to preserve gains, as repigmentation can fade without continued . Other dermatological applications include PUVA (psoralen plus UVA) for , where oral or bath sensitizes skin to UVA (320-400 nm), achieving complete response in 70-80% of early-stage patients after 20-30 sessions by targeting malignant T-cells. For prevention, (PDT) using topical photosensitizers like methyl aminolevulinate activated by red light effectively clears subclinical lesions and reduces new occurrences by 50% or more over 12 months in areas. Individualized dosing in these therapies relies on determining the minimal dose (MED), the lowest UV exposure causing faint redness 24 hours post-irradiation, to tailor starting doses at 50-70% of MED and avoid burns while maximizing .

Other Medical Conditions

Light therapy has been applied to various non-psychiatric and non-dermatological conditions, leveraging specific wavelengths to target physiological processes such as , mitochondrial function, and metabolic synthesis. These applications focus on internal systemic or ocular issues, demonstrating in reducing treatment burdens and improving outcomes through targeted light exposure. In , phototherapy using blue-green in the 450-500 nm range facilitates the of , converting unconjugated into water-soluble isomers that can be excreted without hepatic conjugation. The primary mechanism involves the absorption of by native Z,Z-, leading to configurational to Z,E- and E,Z-, as well as structural to lumirubin, a more polar and rapidly excretable product. This process occurs in the skin's bed, where penetrates to interact with circulating . Phototherapy significantly reduces the need for , with studies indicating efficacy in preventing severe hyperbilirubinemia and lowering exchange rates from historical levels of 20-30% to approximately 2-5%, representing an about 80-90% reduction in such interventions. For retinal conditions like age-related (), low-level red and near-infrared (NIR) light therapy, typically at 670-810 nm, promotes mitochondrial rescue in retinal pigment epithelial cells and photoreceptors. This photobiomodulation enhances ATP production, reduces , and improves , leading to better in early-stage dry AMD patients. Clinical trials have shown modest improvements in best-corrected visual acuity, with some participants gaining 5-10 letters on eye charts after repeated sessions. Limited evidence suggests potential benefits of low-level red light therapy for other eye conditions beyond aging and dry AMD, including diabetic retinopathy, glaucoma, and myopia control, particularly in children via repeated sessions; however, results are mixed and require ongoing treatment. For diabetic retinopathy, animal studies indicate that photobiomodulation using far-red light (670 nm) can mitigate diabetes-induced retinal abnormalities, such as oxidative stress and inflammation, through direct and indirect mechanisms, even when initiated weeks after diabetes onset. Emerging research points to neuroprotective effects in glaucoma, where red light may reduce inflammation, enhance antioxidant reactions, and promote retinal ganglion cell regeneration, potentially synergizing with existing treatments. For myopia control in children, meta-analyses of randomized controlled trials show that repeated low-level red light (650 nm) therapy slows axial length elongation and refractive progression over short-term periods (3-12 months), with mean differences in axial length of -0.16 to -0.31 mm compared to controls, though long-term safety, efficacy, and potential rebound effects after cessation need further investigation. Ultraviolet B (UVB) light therapy at 290-315 nm is used to treat in sunlight-deprived populations, such as the elderly, institutionalized individuals, or those in high-latitude regions with limited winter sun. UVB exposure converts in the skin to previtamin D3, which isomerizes to vitamin D3 for systemic absorption. Dosing is calibrated to equivalents of 15-20 minutes of midday summer sun exposure on fair (without ), typically 2-3 times weekly, to achieve serum 25-hydroxyvitamin D levels above 50 nmol/L, avoiding overexposure risks. Red and near-infrared light therapy aids and reduces pain and inflammation in musculoskeletal conditions, including arthritis. By stimulating synthesis, fibroblast proliferation, and remodeling, it accelerates formation and tensile strength recovery in chronic wounds, with clinical trials showing 40-50% faster healing rates. For conditions like osteoarthritis and rheumatoid arthritis, sessions at 630-830 nm reduce joint pain and inflammation by 20-50%, via mechanisms such as endorphin release, improved , and cytokine modulation. Preliminary evidence also suggests benefits for dementia symptoms, with small studies indicating improvements in cognitive function through reduced neuroinflammation and enhanced mitochondrial function in brain cells, though larger trials are required. Results for these applications vary, and more large-scale studies are needed to confirm efficacy. There is limited support for red light therapy in weight loss, cellulite reduction, cancer treatment, or broad mental health benefits. As an adjunct in cancer care, low-level light therapy serves a palliative role in managing oral induced by , using red or NIR wavelengths (630-660 nm or 780-830 nm) to mitigate and promote mucosal repair. Preventive applications before cycles can reduce mucositis severity by up to 40%, decreasing pain and opioid needs while improving oral intake . Evidence supports its use for oral pain conditions like burning mouth syndrome, with moderate certainty for symptom relief. Low-level light therapy (LLLT), particularly using near-infrared light at 830 nm, has shown promise in treating Hashimoto's thyroiditis, also known as chronic autoimmune thyroiditis (CAT). Clinical studies indicate that LLLT can reduce the required dosage of levothyroxine (LT4), decrease thyroid peroxidase antibody (TPOAb) levels, improve thyroid echogenicity, and potentially reduce autoimmune activity. A 2010 pilot study involving 15 patients found a significant reduction in LT4 dosage from 96 ± 22 μg/day to 38 ± 23 μg/day after 9 months, with 47% of patients not requiring LT4, and decreased TPOAb levels from 982 ± 530 U/ml to 579 ± 454 U/ml. A 2012 randomized, placebo-controlled trial with 43 patients confirmed these benefits, showing a lower mean LT4 dose (38.59 ± 20.22 μg/day vs. 106.88 ± 22.90 μg/day in placebo) and reduced TPOAb levels. A 2022 review further supports LLLT's role in reducing thyroid inflammation, inhibiting immune cell trafficking, modulating inflammatory responses, and promoting thyroid regeneration.

Therapeutic Techniques

Bright Light Therapy

Bright light therapy involves exposure to high-intensity visible , typically in the 400-700 nm range, to mimic natural daylight and influence circadian rhythms. Devices deliver measured in , with standard protocols emphasizing morning sessions to promote phase advances in the sleep-wake cycle. This non-invasive technique is commonly applied at home, using specialized equipment designed for safety and efficacy. Light boxes are the primary devices for bright light therapy, providing 10,000 of full-spectrum, UV-free illumination at a of 12 to 18 inches from the user's face to ensure therapeutic dosing without discomfort. These units typically feature a of around 5,000 K to replicate cool daylight, and they are positioned above , angled downward to allow indirect exposure while the user engages in activities like reading or working. Placement above helps simulate natural overhead , optimizing light entry through the eyes' lower . Dawn simulators and portable devices offer convenient alternatives for home use, particularly in treating (SAD), by gradually ramping up light intensity to mimic sunrise. Dawn simulators increase from near-darkness to several hundred over 30 to before waking, easing the transition to alertness without abrupt alarms. Portable versions, such as compact lamps or wearable light visors, allow flexibility for travel and maintain similar gradual intensity profiles, often reaching 10,000 at close range for shorter sessions. Standard protocols recommend morning exposure shortly after waking to achieve circadian phase advances, with sessions lasting 20 to 60 minutes depending on device intensity. For instance, 10,000 lux requires about 30 minutes, while lower intensities like 2,500 lux may extend to 1-2 hours. Seasonal adjustments are tailored for conditions like SAD, initiating therapy in early fall and tapering in spring as natural daylight increases, to align with symptom patterns. Brief applications also support sleep disorders by advancing sleep phase timing, as detailed in related clinical sections. Accessories enhance safety and adherence, including UV filters that block ultraviolet wavelengths to prevent eye and skin exposure while permitting visible light transmission. Compliance monitoring apps, integrated with some devices, track session duration, timing, and frequency through user logs or automated reminders, improving treatment outcomes by ensuring consistent use. Lux measurements follow Illuminating Engineering Society of North America (IESNA) standards for , quantifying light flux per unit area in lumens per square meter to verify device output uniformity and efficacy. Therapeutic efficacy often incorporates energy dose calculations, expressed as lux-hours (illuminance multiplied by exposure time), to standardize the total light exposure required for phase-shifting effects.

Ultraviolet Phototherapy

Ultraviolet phototherapy utilizes specific wavelengths of to treat various conditions, primarily through photochemical reactions in the skin. Broadband UVB (BB-UVB), spanning 280-320 nm, employs fluorescent lamps that emit a wider of UVB , while UVB (NB-UVB) focuses on a peak emission at 311 nm using specialized TL-01 lamps, which are more effective for clearing with fewer side effects and longer remission periods. Full-body exposure is typically achieved using walk-in cabinets that encircle the patient, ensuring uniform irradiation of the entire surface with multiple lamps arranged in panels for efficient treatment sessions lasting minutes. PUVA, or psoralen plus UVA, combines photosensitizing psoralen drugs with UVA radiation (320-400 nm) to enhance therapeutic efficacy. Oral methoxsalen is dosed at 0.4-0.6 mg/kg body weight, taken 1-2 hours prior to UVA exposure, while topical psoralen is applied directly to affected areas for localized treatment; this protocol induces DNA cross-linking in skin cells, suppressing abnormal proliferation. Treatments occur 2-3 times weekly, with UVA doses starting at 1-2 J/cm² and incrementing based on response to avoid excessive erythema. Clinical units, often large cabinets in clinics, provide supervised full-body sessions, whereas home units include compact panels or handheld devices for targeted areas like hands or , allowing convenient after initial clearing. schedules typically involve 2-3 sessions per week to sustain remission, with home devices requiring patient training and periodic to ensure accurate dosing. Monitoring during UV phototherapy involves assessing skin response using erythema grading scales, such as grade 1 (mild confined to exposure sites) to grade 4 (severe, painful with bullae), to adjust doses and prevent burns. Cumulative dose limits, particularly for PUVA in , are recommended not to exceed 1000 J/cm² over a lifetime to minimize long-term risks like . Additionally, the action spectrum for production in skin peaks at approximately 295 nm, where UVB efficiently converts to previtamin D3, supporting treatments for deficiency-related conditions.

Photodynamic and Photothermal Therapies

Photodynamic therapy (PDT) is a light-activated treatment that employs , light, and molecular oxygen to induce selective cell death, primarily through the generation of . In this process, a such as (ALA), a that leads to the accumulation of in target cells, is administered topically or systemically and allowed to localize in diseased tissues. Upon illumination with red light at approximately 630 nm, the absorbs photons and transitions from its ground to an excited , followed by to a longer-lived , as illustrated in the of energy levels. The then transfers energy to ground-state oxygen via a type II photochemical reaction, producing cytotoxic that damages cellular components like membranes, proteins, and DNA, ultimately leading to or in an oxygen-dependent manner. Typical protocols for ALA-based PDT involve incubation times of 0.5 to 4 hours to allow accumulation, followed by a drug-light interval of about 3 hours before with doses ranging from 37 to 200 J/cm² at fluence rates of 11–14 mW/cm², depending on the clinical application and formulation. For instance, in the treatment of superficial , topical ALA-PDT has demonstrated complete response rates of 80–95% at short-term follow-up, offering a non-invasive alternative with favorable cosmetic outcomes. Antimicrobial PDT, another key application, utilizes like activated by 660 nm to generate that disrupts microbial cell walls and membranes, effectively inactivating bacteria, fungi, and viruses such as without promoting resistance. Photothermal therapy (PTT), in contrast, relies on light-absorbing agents to convert photonic energy into localized heat for therapeutic destruction, distinct from the photochemical oxidative damage in PDT. Gold nanoparticles, particularly nanorods or nanoshells, are widely used due to their strong in the near-infrared (NIR) range of 700–1100 nm, which penetrates tissues deeply with minimal absorption by water or hemoglobin. When irradiated with an NIR (e.g., 808 nm at 0.3–2 W/cm² for 3–10 minutes), these nanoparticles efficiently convert over 50% of the absorbed light into heat, raising local temperatures above 42°C to induce , protein denaturation, and tumor through or while sparing surrounding healthy tissue. In cancer applications, such as or models, targeted gold nanorod conjugates have achieved intratumoral temperatures of 60–67°C, significantly reducing tumor volume and enhancing survival rates in preclinical studies.

Low-Level Light Therapy

Low-level light therapy (LLLT), also known as photobiomodulation (PBM), utilizes low-intensity light sources to deliver non-thermal to cells and tissues, promoting repair, reducing , and alleviating without ablating or heating the target area. This therapy typically employs wavelengths in the (630–670 nm, commonly 630-660 nm) or near-infrared (780–950 nm, commonly 800-850 nm) , with power densities ranging from 1 to 100 mW/cm² to ensure safe, non-damaging energy delivery; these wavelengths enable light penetration into tissues (up to several millimeters for red light and deeper for near-infrared), reduce inflammation by modulating cytokines, improve blood flow through vasodilation and angiogenesis—which promotes increased delivery of oxygen, nutrients, and fluid, potentially causing temporary swelling and fullness similar to a resistance training pump, observed acutely after sessions or with consistent use—and stimulate cellular repair via enhanced mitochondrial activity. Common devices for LLLT include Class IIIb lasers, such as gallium-aluminum-arsenide (GaAlAs) diode lasers, and light-emitting diode (LED) panels or arrays, which provide coherent or incoherent light for targeted applications like , pain relief, and dermatological conditions such as acne, skin rejuvenation, increased collagen production, improved elasticity, reduction of redness and scars, and treatment of psoriasis. Effective red light therapy devices are distinguished from ordinary red lights by their engineered parameters, including specific narrow wavelengths, sufficient irradiance (50–200 mW/cm²), and focused beams for optimal tissue penetration; trusted devices further incorporate evidence-based parameters with irradiance verified through independent or third-party testing, high build quality, extended warranties, and consistent positive feedback from experts and users, while low-cost unbranded devices should be avoided as they frequently underdeliver on power output; ordinary red lights provide minimal to no therapeutic benefit, as confirmed by experts and reviews. In dermatological uses, LED-based devices including face masks have FDA clearance for treating mild to moderate acne and reducing wrinkles, with clinical studies demonstrating improvements in skin texture, tone, acne clearance, and wrinkle reduction through consistent use of 3–5 sessions per week for 4–12 weeks; results vary by individual factors such as skin type and routine adherence. Emerging research also indicates potential applications in promoting hair regrowth in androgenetic alopecia, with FDA-cleared devices available, as well as ophthalmic conditions beyond age-related macular degeneration, including limited preclinical evidence for mitigating diabetic retinopathy by reducing oxidative stress and inflammation in retinal cells, promising but limited animal model evidence for glaucoma through enhanced mitochondrial function and reduced retinal ganglion cell damage, and clinical evidence from randomized trials and meta-analyses showing efficacy in slowing myopia progression in children via repeated low-level red light sessions (typically twice daily), though results are mixed with some studies showing no significant benefits and long-term safety requiring further investigation. These devices operate at low power outputs, typically below 500 mW, to maintain the non-thermal nature of the treatment. Treatment protocols in LLLT can use either (CW) or pulsed wave (PW) modes, with pulsed frequencies often ranging from 2 to 8000 Hz to potentially enhance tissue penetration and cellular recovery while minimizing any minor buildup. Recommended session durations typically start at 5-10 minutes and build up to 10-20 minutes, performed 3-5 times per week at a distance of 6-12 inches from the device to ensure optimal irradiance. Total per session generally falls between 4 and 50 J/cm², adjusted based on tissue depth—lower for superficial targets (1–10 J/cm²) and higher for deeper ones (10–50 J/cm²)—to optimize biostimulatory effects. Noticeable results from red light therapy, such as improvements in skin texture and reduced wrinkles, typically appear after 4-12 weeks of consistent use. The primary mechanism of LLLT involves light absorption by , a key enzyme in the mitochondrial , which accelerates electron flow, boosts ATP production, and elevates mitochondrial membrane potential. This process also generates low, non-damaging levels of (ROS) that serve as signaling molecules, activating transcription factors like to upregulate genes for , migration, and anti-inflammatory responses, while further contributing to reduced inflammation, improved blood flow, and stimulated cellular repair. LLLT adheres to the Arndt-Schulz rule, which posits a biphasic dose-response curve: low doses stimulate vital cellular activities, moderate doses achieve peak efficacy, and high doses inhibit or suppress responses, guiding parameter selection to avoid inhibitory thresholds. For instance, energy densities around 3–5 J/cm² often promote tissue repair, while 50–100 J/cm² may diminish benefits. Evidence supports several applications of red light therapy within LLLT, including reduction of joint pain, muscle soreness, and symptoms of arthritis using near-infrared wavelengths of 800–900 nm, fluence of 10–50 J/cm², and power density of 30–100 mW/cm², with sessions of 10–30 minutes administered 3–5 times per week, achieving pain reduction.—with cumulative enhancements such as faster muscle recovery, reduced soreness/inflammation, and improved endurance becoming noticeable after 1–4 weeks of regular use (quicker recovery within 1–2 weeks, deeper benefits by 3–4 weeks)—as well as aiding wound healing, though results vary and more large-scale studies are needed. Preliminary evidence suggests benefits for alleviating dementia symptoms and oral pain, such as in mucositis, but support is limited for weight loss, cellulite reduction, cancer treatment, or broad mental health benefits.

Safety and Risks

Adverse Effects by Wavelength

Adverse effects of light therapy vary significantly by wavelength, with (UV) posing the most substantial risks to and eyes due to its high energy and ability to induce photochemical damage. In UV phototherapy, particularly narrowband UVB (NB-UVB) and psoralen plus UVA (PUVA), acute effects include , which manifests as redness and resembling sunburn, occurring when exposure exceeds the minimal erythema dose (MED). Chronic UV exposure contributes to , characterized by premature wrinkling, elastosis, and pigmentary changes from cumulative damage to dermal and . Furthermore, elevates the risk of , with (SCC) incidence increasing up to 14-fold compared to low-exposure patients and over 30-fold relative to the general population after more than 150-200 treatments, due to psoralen's photosensitizing effects amplifying DNA damage. Ocular exposure to UV during therapy can lead to , as UVB (280-315 nm) promotes lens protein and opacification, with studies linking prolonged artificial UV to cortical and subcapsular formation. Visible light, especially in the blue spectrum (400-500 nm), carries risks primarily to the and in bright light therapy applications. Retinal phototoxicity, known as the blue light hazard, arises from photochemical reactions generating that damage photoreceptors and , potentially leading to macular degeneration-like changes detectable via testing as central scotomas or distortions. High-intensity visible light exposure can also induce headaches or trigger migraines in susceptible individuals, with reports indicating exacerbation of symptoms through activation and cortical hyperexcitability. These effects are dose-dependent, with brighter intensities (e.g., >2,500 ) correlating with higher incidence during sessions for . Infrared (IR) and near-infrared (NIR) wavelengths (700 nm-1 mm), used in low-level light therapy, primarily cause effects at high doses, leading to burns if exposure exceeds safe thresholds, as accumulation denatures proteins in and deeper tissues. Prolonged IR sessions can result in , as elevated tissue temperatures increase and fluid loss, particularly in non-cooled devices. NIR penetration may also accelerate activity, mimicking UV-induced in . However, at the low power densities used in low-level light therapy, including full-body applications combining visible red light (600-700 nm) and NIR, adverse effects are generally minimal when used as directed. These therapies emit no harmful UV rays, with rare mild side effects such as temporary redness or eye strain if eyes are unprotected; clinical studies report no serious adverse events, and many devices are FDA-cleared for specific uses, indicating non-invasive and well-tolerated profiles. Patients with photosensitive conditions, darker skin tones (which may experience pigmentation changes with overexposure), or those on certain medications should consult a healthcare provider. Adverse effects across wavelengths are inherently dose-dependent, with UV risks scaling with the UV index or cumulative fluence; for instance, the MED for UVB in fair skin types ranges from 200-600 mJ/cm², beyond which and DNA damage intensify. Safe exposure limits are outlined in International Commission on Protection (ICNIRP) guidelines, which specify maximum permissible exposure (MPE) values by wavelength band—for UV, thermal and actinic limits prevent (e.g., 30 J/m² for UVB over 8 hours); for visible light, the blue light hazard MPE is a radiance limit of 100 W/m²/sr for wavelengths 300-700 nm and exposures >10,000 s to avoid damage; this equates to corneal below approximately 1 W/m² (0.1 mW/cm²) for typical extended sources; and for IR-A (700-1400 nm), 10 mW/cm² limits thermal injury. These guidelines emphasize protective measures like and timed sessions to mitigate wavelength-specific harms.

Contraindications and Monitoring

Light therapy requires careful patient screening to identify contraindications and implement appropriate monitoring protocols to minimize risks. Absolute contraindications include systemic lupus erythematosus, , and active , as these conditions can be exacerbated by light exposure due to inherent . Pregnancy represents an absolute contraindication specifically for psoralen plus ultraviolet A (, owing to potential teratogenic effects on the . Relative contraindications encompass the use of photosensitizing medications, such as tetracyclines, which heighten the risk of phototoxic reactions during treatment. Individuals with should avoid therapies involving flashing or , as these may precipitate seizures in photosensitive patients. Monitoring begins with baseline assessments to establish safety parameters. Eye examinations are recommended prior to initiating therapy for patients with preexisting conditions to evaluate vulnerability to . Skin type evaluation using the guides initial dosing to prevent burns in fair-skinned individuals. For PUVA, regular blood tests assessing liver function, including levels, are essential to detect early signs of during long-term use. levels may warrant periodic monitoring in patients with known deficiency, given UV exposure's role in synthesis. Therapeutic adjustments are tailored to patient factors for optimal safety. Elderly individuals often require dose tapering and more frequent increments due to reduced skin resilience and higher likelihood of concurrent photosensitizing medications. When combining light therapy with retinoids, doses should be reduced by one-third to one-half to mitigate amplified . In patients with , the guidelines highlight the need for vigilant monitoring, as light therapy carries a risk of induction in approximately 10-15% of cases.

History and Research

Historical Development

The use of , known as heliotherapy, dates back to ancient civilizations, including , where it was employed to treat various skin conditions such as and other dermatological ailments. In ancient and , similar practices were documented for balancing bodily humors and addressing skin diseases. The modern era of light therapy began in the late 19th century with the work of Danish physician Niels Ryberg Finsen, who pioneered the use of concentrated artificial light to treat , a form of cutaneous . Finsen's development of carbon arc lamps to deliver filtered (UV) rays marked a significant advancement, demonstrating that specific wavelengths could inhibit bacterial growth without relying solely on natural sunlight. For his contributions, Finsen was awarded the in Physiology or Medicine in 1903. In the early , artificial light sources gained prominence in sanatoriums, where patients with and other conditions received treatments using UV-emitting lamps to mimic heliotherapy indoors. By the 1910s, facilities like the in the United States integrated carbon arc and incandescent lamps into routines for conditions including and skin infections. These lamps were also applied to , a disorder prevalent in industrialized areas due to limited sun exposure; exposure to UV light from carbon arc lamps was shown to promote healing by facilitating production in the skin, a link established through experiments in the 1920s. A key milestone in dermatological applications came in 1925 with the Goeckerman regimen, developed by American dermatologist William H. Goeckerman, which combined crude application with UVB exposure from hot quartz lamps to treat severe , achieving high clearance rates and influencing subsequent phototherapy protocols. Following , advancements in lamp technology led to the adoption of fluorescent tubes emitting broadband UVB in the , providing a safer and more efficient alternative to earlier arc lamps for treating skin disorders like and eczema in clinical settings. The 1980s saw the expansion of light therapy into psychiatric applications, with psychiatrist Alfred J. Lewy demonstrating that bright artificial light (approximately 2,500 ) could suppress production and shift circadian rhythms, offering a non-pharmacological treatment for (SAD). Building on this, psychologist Michael Terman advanced clinical protocols for SAD, showing through controlled trials that morning exposure to bright light improved depressive symptoms in up to 60% of patients by aligning internal clocks with daylight cycles.

Current Research and Future Directions

Recent studies from the 2020s have explored photobiomodulation (PBM) for symptoms, particularly and brain fog. A 2023 open-label pilot study with 14 participants reported that transcranial and whole-body PBM led to symptom relief, with 100% achieving normal or improved cognitive function. Larger randomized controlled trials (RCTs) are needed to confirm these findings. Regulatory advancements have facilitated broader access to light therapy. The U.S. (FDA) has issued clearances for several home-use PBM devices since 2020, including low-level light therapy systems for and skin conditions. The (WHO) continues to endorse phototherapy as a standard, safe intervention for neonatal hyperbilirubinemia, emphasizing its role in global guidelines for jaundice management in resource-limited settings. Emerging research addresses key gaps in light therapy applications, such as wearable devices, AI-optimized dosing, and integration with (VR) for psychiatric conditions. Wearable LED-based PBM systems have shown promise in home-care settings, enabling continuous transcranial stimulation to improve cognitive symptoms in neurodegenerative disorders, with ongoing trials evaluating their portability and adherence. AI algorithms are being developed to personalize light dosing in photomedicine by analyzing patient-specific factors like type and circadian rhythms. In psychiatry, VR-integrated light therapy, such as immersive color-based environments, has reduced anxiety levels in preliminary 2025 studies by combining sensory exposure with targeted wavelengths. Future directions include optogenetics-inspired therapies, transcranial LED-based photobiomodulation for neurodevelopmental disorders, and nanotechnology-enhanced photodynamic therapy (PDT). Optogenetic approaches, adapted for non-invasive human use, are advancing vision restoration in retinal diseases by sensitizing surviving neurons to light, with clinical trials reporting improved visual acuity in macular degeneration patients. Transcranial photobiomodulation has demonstrated reductions in core autism spectrum disorder (ASD) symptoms like social withdrawal in small 2025 cohorts. Nanotechnology in PDT is revolutionizing targeted cancer treatments by improving photosensitizer delivery and tumor penetration, with recent nanocarrier systems achieving up to 90% efficacy in preclinical models for solid tumors. Additionally, 2023-2024 studies on 40 Hz blue light stimulation have revealed its role in enhancing amyloid-beta clearance via glymphatic pathways, with 2025 research extending to multisensory gamma stimulation showing reduced amyloid burden in Alzheimer's models. These innovations highlight evidence gaps in long-term safety and scalability, directing future research toward multimodal, personalized protocols.

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