Light Therapy

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1. What Light Therapy Is and How It Is Applied

Light therapy refers to targeted exposure to artificial or natural light of defined wavelength, intensity and duration. It is used as a non-invasive procedure to influence physiological processes via the retina or skin. Devices range from daylight lamps through LED panels to low-intensity laser sources. Administration usually occurs on an outpatient basis or at home, with standardised protocols for exposure duration (minutes to hours), time of day and repetition frequency.

Crucial for the interpretation of the studies is the wavelength-specific effect: blue light (460–480 nm) suppresses melatonin release and shifts the circadian rhythm, while red light (600–700 nm) and near infrared primarily trigger local tissue effects such as increased perfusion or mitochondrial activation. UV radiation acts immunomodulatorily but carries carcinogenic risks. Dosimetry (light dose in joules/cm²) is inadequately documented in many studies, which complicates comparisons between works. In addition, controls for placebo effects are often lacking, as light exposure is difficult to blind. Individual light sensitivity varies greatly, depending on genetics, skin pigmentation and pre-existing eye diseases.

2. What the Studies Show in Detail

The supporting human studies and the most important reviews — by application area, with population, duration, preparation, measurement variable and result as reported by the original studies. Numbers appear only where the study states them.

Distribution of controlled human studies by application area — the numbers are not additive, as one study may be assigned to several areas:

Safety and Tolerability

Primary endpoints were changes in axial length (AL) and spherical equivalent (SER). Adverse events were mild and included transient glare or dry eyes, without serious complications. The study was limited to 12 months, and preclinical investigations (e.g. in animal models) showed effects on retinal structure whose transferability to humans has not been examined 1.

A prospective, randomised intra-individual study with 27 patients (26 completed) with non-segmental vitiligo on the hands compared the combination of fractional CO₂ laser, narrowband UVB phototherapy (NB-UVB) and topical clobetasol propionate (group A) with NB-UVB and clobetasol propionate alone (group B). Group A received 10 sessions of fractional CO₂ laser at weekly intervals as well as 20 NB-UVB sessions (twice weekly). After 12 weeks, 23.1 % of lesions in group A showed good to excellent repigmentation, compared with 3.9 % in group B (p = 0.065). The mean improvement in repigmentation was higher in the combination group. The study examined exclusively the effect of combination therapy, not of light therapy alone, and was restricted to difficult-to-treat areas (hands). The case number was small, which limits statistical power.

A systematic review with meta-analysis compared the efficacy and safety of 308-nm excimer lamps with excimer lasers and NB-UVB in vitiligo. Six studies with 411 patients (764 lesions) were included. There was no significant difference in ≥50% repigmentation between excimer lamp and excimer laser (RR = 0.97; 95% CI: 0.84–1.11) or NB-UVB (RR = 1.14; 95% CI: 0.88–1.48). Results were also comparable for ≥75% repigmentation. Adverse effects such as itching or burning were mild and did not lead to treatment discontinuation. The studies were heterogeneous with regard to treatment protocols and follow-up duration, which limits comparability.

A retrospective multicentre cohort study in 5- to 14-year-old children with myopia examined the effects of RLRL on retinal cone density using adaptive optics scanning laser ophthalmoscopy (AOSLO). Cone density was measured along 4 retinal meridians. After 12 months there was no significant change in cone density compared with the control group (n = 38). Fundus changes or structural anomalies were not observed. The study was retrospective and had a short follow-up period, so long-term effects on photoreceptors remain unclear 16.

Hypothesis The available data suggest that RLRL is safe and well tolerated in children with myopia provided the device parameters (wavelength, intensity, application duration) are observed. The mild adverse effects (e.g. glare) are reversible and do not lead to treatment discontinuation. However, long-term data on retinal safety are lacking, particularly for high-frequency applications over several years.

Cognition and Psyche

A randomised controlled trial with 49 adolescents (14.6 ± 1.0 years) with delayed sleep phase disorder (DSPD) compared cognitive behavioural therapy plus light therapy (CBT + BLT) with a wait-list control. After 6 weeks the CBT + BLT group showed moderate to large improvements (effect sizes d = 0.65–1.24) in sleep latency, sleep onset time, wake-up time and total sleep time on school days. The effects remained stable over 6 months. The study was limited to adolescents with DSPD, and compliance with light therapy was not measured objectively 2.

A 9-week randomised study with 75 adults with major depression (DSM-IV) examined a chronotherapeutic intervention (wake therapy + light therapy + sleep rhythm stabilisation) compared with daily exercise. All patients received duloxetine. The study was open-label and the control group (exercise) was less intensive, so the specific effect of light therapy as part of the combination cannot be isolated 6.

A systematic review and meta-analysis of 11 randomised controlled trials with 858 patients (75.6% female) examined the efficacy of light therapy (BLT) as adjunctive treatment in non-seasonal depression. The effects were moderate (SMD = −0.45 for depression scales) and provide an indication of possible efficacy as adjunctive therapy.

BLT led to a significant reduction in depressive symptoms (SMD = −0.62, p < 0.001), particularly with 2–5 weeks of use (SMD = −0.78) and as monotherapy (SMD = −0.71). In perinatal depression the effect was not significant (SMD = −0.17). The studies showed moderate heterogeneity and most lacked an active control group 14.

An observational study with 141 bipolar patients with treatment-resistant depression examined the combination of lithium, sleep deprivation and light therapy. After 1 week 70% achieved a 50% reduction in HAM-D scores and suicidality decreased significantly. The study was uncontrolled and the combination therapy does not allow conclusions about the effect of light therapy alone 15.

Hypothesis Controlled studies of light therapy show antidepressant effects in non-seasonal depression, particularly in combination with antidepressants. Efficacy appears to be dose-dependent (higher lux values and longer duration of use lead to stronger effects). In bipolar patients an uncontrolled study with combination therapy (lithium, sleep deprivation, light therapy) provides an indication of possible effects provided mania risk is controlled by mood stabilisers. The effect of light therapy alone was not examined. Data on perinatal depression are limited and further studies with active controls are still required.

Myopia Control and Ocular Parameters

The RLRL group showed a maximum mCT increase of 14.755 µm after 1 month, which decreased to 1.543 µm by month 12 (control group: 0.38 µm, p < 0.001). The study examined the association of the transient choroidal thickening with myopia progression; statements about visual acuity cannot be derived from this 3.

After 12 months the RLRL group showed a significantly smaller AL increase (0.01 mm vs. 0.39 mm, p < 0.05) and SER worsening (−0.05 dpt vs. −0.64 dpt, p < 0.05). Subfoveal choroidal thickness increased in the RLRL group as a preclinical parameter, while accommodative function remained unchanged. After discontinuation of therapy a rebound effect occurred (AL increase of 0.16 mm in 3 months). The study was limited to 12 months and compliance was not measured objectively 4.

A randomised controlled trial with 62 children (7–15 years) compared RLRL (650 nm, 3 minutes twice daily) with 0.01% atropine eye drops over 12 months. The RLRL group showed a significantly smaller AL increase (0.08 mm vs. 0.33 mm, p < 0.001) and SER worsening (−0.03 dpt vs. −0.60 dpt, p < 0.001). 53.2% of RLRL patients had an AL increase < 0.1 mm compared with 9.7% in the atropine group. The study was single-blind and compliance with RLRL therapy was not controlled 5.

After 12 months 21.85% of RLRL patients showed AL shortening > 0.05 mm, 15.13% > 0.10 mm and 5.88% > 0.20 mm compared with 0% in the control group (p < 0.001). Mean AL shortened by 0.156 mm in the RLRL group. Younger children (< 10 years) showed AL shortening more frequently. The study was not primarily designed for this endpoint and the clinical relevance of AL shortening is unclear 7.

A randomised study with 94 pre-myopic children (−0.50 dpt < SER ≤ +0.75 dpt) examined the effects of RLRL on choroidal and retinal structure. After 12 months the RLRL group showed a significant increase in foveal choroidal thickness (maximum +19.58 µm after 3 months) and choriocapillary perfusion area. AL remained stable while it increased in the control group. The study was limited to pre-myopic children and long-term effects on retinal structure are unknown 8.

A randomised controlled trial with 50 children (8–12 years) compared photobiomodulation (PBM, 650 nm, 3 minutes twice daily) with single-vision lenses. The study in children showed no significant changes in subfoveal choroidal thickness or other ocular parameters; the longer-term effect was not conclusively examined.

Hypothesis RLRL and related procedures such as PBM provide indications of effects in myopia control, particularly through slowing of AL increase and transient choroidal thickening. The effects appear to be dose-dependent and compliance is a critical factor. The observed AL shortening under RLRL is a novel finding whose clinical significance is nevertheless unclear. Long-term data on safety and efficacy over several years are still required, as are further direct comparisons with established therapies such as atropine or orthokeratology.

Oncology

A randomised controlled trial with 81 cancer patients with cancer-related fatigue (CRF) examined the efficacy of light therapy. The study was blinded, but compliance with light therapy was not measured objectively and follow-up duration was short.

A review on sleep disorders in cancer patients recommends light therapy as a possible intervention, particularly in combination with cognitive behavioural therapy or melatonin. The evidence is based on small studies with heterogeneous designs, and most recommendations for hypnotics derive from the treatment of primary insomnia. Specific studies on the efficacy of light therapy in cancer patients with sleep disorders are lacking.

A systematic review on acute radiodermatitis (ARD) identified photobiomodulation (PBM) as a promising intervention based on several randomised controlled studies. The quality of the evidence was overall low and further studies with standardised protocols are necessary.

Hypothesis Light therapy shows potential in the treatment of cancer-related fatigue, yet specific studies with longer follow-up periods are still required. For sleep disorders in cancer patients specific data on the efficacy of light therapy are lacking and the recommendations are based on extrapolations from other populations. Standardised protocols and larger controlled studies are necessary to clarify the role of light therapy in oncology.

Fatigue and Stress

A randomised controlled trial with 193 adolescents (16.3 ± 0.4 years) examined school-based sleep interventions with light therapy (BLT) and/or parental involvement (PI). The BLT group received one weekend of phase advance using green LED glasses (500 nm, 506 lux).

A randomised controlled trial with 40 patients with uraemic pruritus compared narrowband UVB (NB-UVB, 3× weekly for 6 weeks) with UVA controls. The study was single-blind, and the sample size was small.

Hypothesis Light therapy (particularly NB-UVB) provided indications of efficacy in some controlled studies for uraemic pruritus, while the effects in adolescent sleep problems are less clear. The combination with other interventions (e.g. sleep education) could increase efficacy, yet controlled studies with active controls and objective measurements (e.g. actigraphy) are still pending.

Skin Disorders

A meta-analysis of 7 randomised controlled trials with 240 patients (413 lesions) compared NB-UVB monotherapy with NB-UVB plus topical calcineurin inhibitors or vitamin D analogues in vitiligo. No significant difference was found in ≥50 % or ≥75 % repigmentation, yet combination therapy was superior for lesions on the face and neck (RR = 1.40 for ≥50 % repigmentation).

A review article on phototherapy in dermatological disorders summarises the evidence for various forms (NB-UVB, PUVA, UVA1). It shows that NB-UVB is the most common form among the therapies examined and demonstrates efficacy in psoriasis, atopic dermatitis and vitiligo. Combination with topical or systemic therapies can enhance the effects, yet long-term data on carcinogenicity and skin ageing are limited. Compliance is a critical factor, as the therapy must be performed at least 2× weekly over several weeks.

Hypothesis Phototherapy is an established treatment for various skin disorders, particularly in generalised psoriasis and vitiligo. Combination with topical agents can improve efficacy in difficult-to-treat areas. Standardised protocols and direct comparisons with other therapies (e.g. biologics) are still pending.

Inflammatory Processes

A review article on psoriasis discusses, among other topics, the role of phototherapy (particularly UVB) in modulating the local neuroendocrine system. UVB phototherapy showed an increase in serum 25(OH)D levels in controlled studies, yet the specific contribution to psoriasis improvement remains unclear. Topical derivatives can enhance the efficacy of phototherapy without inducing hypercalcaemia. The results of controlled studies are nevertheless contradictory, and further clinical studies are required.

Hypothesis Phototherapy could exert anti-inflammatory effects via vitamin D-dependent and -independent mechanisms, yet clarification of a causal relationship between vitamin D levels and clinical improvement in psoriasis is still pending. Combination with vitamin D analogues or other immunomodulators could have synergistic effects, whereby standardised studies with long follow-up periods would need to be examined.

Bones and Joints

A systematic review on various bone-healing procedures summarises preclinical and clinical data on low-level laser therapy (LLLT). Preclinical studies (animal and cell models) show that LLLT promotes angiogenesis, fracture healing and osteogenic differentiation of stem cells, yet the underlying mechanisms remain unclear. The effects depend on wavelength, energy density and cell type. Clinical studies show promising results, yet the protocols are heterogeneous and the quality of the evidence is low.

Hypothesis LLLT could represent an adjuvant therapy in fracture healing or osteoporosis, whereby large randomised controlled trials with standardised protocols are still pending. The optimal dosage and duration of application need to be clarified, and long-term safety data must be collected.

Metabolism and Myopia Mechanisms

A review article on RLRL in myopia discusses the possible mechanisms, including activation of cytochrome c oxidase (a mitochondrial enzyme) and release of dopamine and nitric oxide. RLRL could improve neuronal energy supply in the retina and thereby inhibit axial eye-length growth. Clinical studies show significant slowing of myopia progression, yet the long-term effects on retinal structure remain unclear 17.

Hypothesis RLRL could inhibit myopia progression via metabolic and neuroprotective mechanisms. Long-term effects on the retina and optic nerve are still pending, as is clarification of the optimal wavelength, intensity and duration of application. Direct comparisons with other myopia therapies (e.g. atropine) are necessary.

Overall Picture

The studies available show that various forms of light therapy (e.g. RLRL, NB-UVB, BLT) have been investigated in different indications. Effects are particularly well documented in sub-areas: myopia control (RLRL in children), non-seasonal depression (BLT) and dermatological disorders such as vitiligo or psoriasis (NB-UVB, excimer lamps). This overview nevertheless encompasses many therapy forms, not only the page topic of light therapy in general. Safety and tolerability proved good in the controlled studies provided device parameters were observed. Adverse effects are mostly mild and reversible (e.g. glare, pruritus, erythema).

The evidence base is limited in many areas: for oncological indications (e.g. tumour-related fatigue, ARD) randomised controlled trials with long follow-up periods are still pending. In cognition and psyche the controlled studies examined are small and heterogeneous with regard to light intensity, duration of application and control conditions. For metabolic disorders (except myopia) and bone healing there are few clinical data, and most recommendations are based on preclinical studies or extrapolations. Systematic long-term data on safety (e.g. retinal damage, skin carcinogenicity) are lacking in almost all areas.

3. How Reliable the Research Is

The research landscape on light therapy is shaped by controlled human studies as the largest designated study group, with the evidence concentrating on three main indications: myopia progression in children, sleep-wake disorders (especially delayed sleep phase and circadian rhythm disorders) and fatigue in cancer patients. Skin disorders (e.g. vitiligo, psoriasis) and inflammatory processes follow with a markedly lower number of studies. Basic research primarily investigates molecular mechanisms such as opsins in the retina, mitochondrial function and cytokine regulation, while observational studies frequently analyse epidemiological associations between light exposure and metabolic parameters or mental disorders. Animal and cell-culture studies play a subordinate role.

The methodological robustness of the data varies greatly between the areas of application. Myopia control in children is best supported by multicentre, randomised studies and long-term follow-ups, with rebound effects after therapy cessation and interindividual differences in efficacy documented. Sleep-wake disorders benefit from chronobiological measurements (e.g. melatonin saliva samples), yet placebo controls are difficult due to the subjective symptomatology. Oncological fatigue shows consistent effects in the studies examined, although often with small case numbers and heterogeneous light protocols. Skin diseases such as vitiligo or psoriasis frequently use combination therapies (e.g. UVB + topical steroids), which complicates monocausal statements on the effect of light. Critical are the lack of standardisation of light sources, dosage and application duration, as well as insufficient consideration of confounders such as concurrent medication or lifestyle factors.

4. What Is Frequently Claimed — and What the Data Support

5. The Most Robust Studies

Ordered by strength of evidence, not by familiarity. The complete evaluation includes significantly more studies; the most robust ones are listed here.

Controlled Studies in Humans

• 1 Effect of Repeated Low-Level Red-Light Therapy for Myopia Control in Children: A Multicenter Randomized Controlled Trial (2022)
• 2 A randomized controlled trial of cognitive-behavior therapy plus bright light therapy for adolescent delayed sleep phase disorder (2011)
• 3 Longitudinal Changes and Predictive Value of Choroidal Thickness for Myopia Control after Repeated Low-Level Red-Light Therapy (2023)
• 4 Low-intensity red-light therapy in slowing myopic progression and the rebound effect after its cessation in Chinese children: a randomized controlled (2023)
• 5 Efficacy Comparison of Repeated Low-Level Red Light and Low-Dose Atropine for Myopia Control: A Randomized Controlled Trial (2022)
• Bright light therapy improves cancer-related fatigue in cancer survivors: a randomized controlled trial (2018)
• Evaluation of novel school-based interventions for adolescent sleep problems: does parental involvement and bright light improve outcomes? (2015)
• 6 A 9-week randomized trial comparing a chronotherapeutic intervention (wake and light therapy) to exercise in major depressive disorder patients treate (2012)
• Narrowband ultraviolet B phototherapy for patients with refractory uraemic pruritus: a randomized controlled trial (2011)
• 7 Axial Shortening in Myopic Children after Repeated Low-Level Red-Light Therapy: Post Hoc Analysis of a Randomized Trial (2023)
• Long-term and short-term effects of insomnia in cancer and effective interventions (2014)
• MASCC clinical practice guidelines for the prevention and management of acute radiation dermatitis: part 1) systematic review (2023)
• 8 The Effects of Repeated Low-Level Red-Light Therapy on the Structure and Vasculature of the Choroid and Retina in Children with Premyopia (2024)
• Effects of a preceding fractional carbon dioxide laser on the outcome of combined local narrowband ultraviolet B and topical steroids in patients with (2016)
• Efficacy and Safety of 308-nm Monochromatic Excimer Lamp Versus Other Phototherapy Devices for Vitiligo: A Systematic Review with Meta-Analysis (2016)
• 9 Photobiomodulation therapy retarded axial length growth in children with myopia: evidence from a 12-month randomized controlled trial evidence (2023)
• Effect of narrow band ultraviolet B phototherapy as monotherapy or combination therapy for vitiligo: a meta-analysis (2017)
• 10 Bright Light Therapy for Nonseasonal Depressive Disorders: A Systematic Review and Meta-Analysis (2025)
• Psoralen-narrowband UVB phototherapy for the treatment of vitiligo in comparison to narrowband UVB alone (2013)
• Effect of a Multimodal Lifestyle Intervention on Sleep and Cognitive Function in Older Adults with Probable Mild Cognitive Impairment and Poor Sleep: (2020)
• Methotrexate/narrowband UVB phototherapy combination vs. narrowband UVB phototherapy in the treatment of chronic plaque-type psoriasis--a randomized s (2010)
• 11 A randomised, controlled clinical trial of narrowband UVB phototherapy for clinically isolated syndrome: The PhoCIS study (2018)
• 12 Bright light therapy versus physical exercise to prevent co-morbid depression and obesity in adolescents and young adults with attention-deficit / hyp (2018)
• 13 Efficacy of low-level laser therapy in patients with lower extremity tendinopathy or plantar fasciitis: systematic review and meta-analysis of randomi (2022)

6. Summary

Light therapy showed effects for three indications in the studies examined: myopia control in children (RLRL slowed AL increase), sleep-wake disorders (blue light stabilised circadian rhythm) and oncological fatigue (moderate effects in studies with small case numbers). In non-seasonal depression it showed antidepressant effects, especially in combination with sleep deprivation. Skin diseases such as vitiligo or psoriasis benefited from UVB phototherapy, although often in combination with topical agents. Safety was given when protocols were followed, with mild, reversible side effects (e.g. glare, dry eyes).

Hypothesis The wavelength-specific effect (blue light for circadian effects, red light/infrared for tissue effects) is mechanistically plausible and could be used in a targeted manner. In myopia, RLRL could represent an alternative to atropine or orthokeratology, provided long-term safety is confirmed. For bipolar depression, light therapy could be a pharmacological adjunct, provided mania risks are controlled. Hypothesis: The observed AL shortening under RLRL is a novel finding whose clinical relevance remains unclear. Standardised protocols and active controls would be necessary to strengthen the evidence. Hypothesis: In oncology, light therapy could play a supportive role in fatigue or radiodermatitis, but specific data for sleep disorders in cancer patients are lacking.

Future studies must clarify long-term safety (especially retinal safety with RLRL) and dose-dependent effects. Direct comparisons with established therapies (e.g. atropine in myopia, antidepressants in depression) are necessary. Standardised protocols (wavelength, dosage, application duration) would improve comparability. Large, blinded studies with active controls are lacking for sleep disorders and oncological fatigue. The clinical significance of AL shortening under RLRL should be investigated in long-term studies. In addition, biomarkers for individual light sensitivity (genetics, pigmentation) should be identified to develop personalised approaches.

7. What Remains Open

• Long-term safety: Are there carcinogenic or ocular risks with decades of use of red light or UV radiation?
• Dose-response relationship: What minimum light dose is required for different indications, and how does it vary interindividually?
• Combination therapies: Does light therapy + medication (e.g. atropine in myopia) act synergistically or antagonistically?