[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.

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.

[14] Some types of phototherapy may be effective in the treatment of polymorphous light eruption, cutaneous T-cell lymphoma[15] and lichen planus.

[19] Studies show that light therapy helps reduce the debilitating and depressive behaviors of SAD, such as excessive sleepiness and fatigue, with results lasting for at least 1 month.

[23] A 2007 systematic review by the Swedish agency SBU found insufficient evidence that light therapy was able to alleviate symptoms of depression or seasonal affective disorder.

[29][30] A meta-analysis by the Cochrane Collaboration concluded that "for patients suffering from non-seasonal depression, light therapy offers modest though promising antidepressive efficacy.

Light exposure administered to the eyes before or after the nadir of the core body temperature rhythm can affect the phase response curve.

[40] 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.

[46] 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.

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'[47] effects on permeability or through photoimmunotherapy.

Photosensitizing agents clinically-approved or undergoing clinical trials for the treatment of cancers include Photofrin, Temoporfin, Motexafin lutetium, Palladium bacteriopheophorbide, Purlytin, and Talaporfin.

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.

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.

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.

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).

[52][57] 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.

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.

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.

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.

[73] According to the American Academy of Ophthalmology, there is no scientific evidence showing that exposure to blue light emitting devices result in eye damage.

[74] 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.

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.

[citation needed] Side effects of light therapy for sleep phase disorders include jumpiness or jitteriness, headache, eye irritation and nausea.

Indian medical literature dating to 1500 BCE describes a treatment combining herbs with natural sunlight to treat non-pigmented skin areas.

[84] Finsen used short wavelength light to treat lupus vulgaris, a skin infection caused by Mycobacterium tuberculosis.

[86] 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.