Sunlight was long known to improve acne, and this was thought to be due to antibacterial and other effects of the ultraviolet spectrum; which cannot be used as a treatment due to the likelihood of skin damage in the long term. However, artificial UV didn't work as well as sunlight.
It was found that some of the visible violet light, present in sunlight, in the range 405-420 nm activates a porphyrin (Coproporphyrin III) in Propionibacterium acnes which damages and ultimately kills the bacteria by releasing singlet oxygen. A total of 320 J/cm2 of light within this range renders the bacteria non viable . This part of the spectrum is just outside the ultraviolet and produces little if any tanning or sunburn.
Application of the light for 3 consecutive days has been shown to reduce the bacteria in the pores by 99.9%. Since there are few porphyrins naturally found in the skin, the treatment is believed safe except in patients with porphyria; although eye protection is necessary due to light sensitive chemicals in the retina. The light is usually created by fluorescent lamps, bright LEDs or dichroic filament bulbs.
Treatment is often accompanied with application of red light which has been shown to activate ATP in human skin cells (essentially a photobiomodulation effect), and seems to improve response rates.
Overall improvements of on average 76% for 80% of patients occurs over 3 months; most studies show that it performs better than benzoyl peroxide and the treatment is far better tolerated. However, approximately 10% of users see no improvement.
Home use light boxes usually work well, are effective for people with long-term acne, are likely to be cheaper than dermatologist office light treatments, and can be repeated over several years for negligible cost, as opposed to once weekly or fortnightly. The light at a dermatologist clinic is likely to be of a higher intensity, however, possibly negating the disadvantage of infrequent use. As of 2007 even though most light boxes are considered expensive, the cost is on a par with the total cost of benzoyl peroxide, moisturiser and facial washes over the total life of the light box.
There is some skepticism and lack of data over some of the treatments of acne vulgaris through visible light, mainly for the newer and relatively experimental photodynamic treatments.
A feature of psoriasis is localised inflammation mediated by the immune system. UV radiation is known to suppress the immune system and reduce inflammatory responses. Light therapy for skin conditions like psoriasis or eczema use UVA (315-400nm waveband) or UVB (280-315nm waveband) light waves. UVA, combined with a drug taken orally, is known as PUVA treatment. Narrow Band UVB is the 310nm wave length and is given as a light therapy treatment rather than full spectrum UVB.
Monochromatic infrared light emitted at a wavelength of 890 nm has been shown effective, through limited clinical studies, to help restore sensation and reduce pain in patients with neuropathy and to improve circulation of non-healing ulcers, thus increasing their healing rate. It is thought that the infrared light helps to release nitric oxide into the bloodstream, which aids in increasing local circulation and improving blood flow. People with diabetes have poor circulation due to their naturally low levels of nitric oxide and sedentary habits.
While full sunlight is preferred for seasonal affective disorder, there are a number of products (such as light boxes) using very intense artificial illumination that are effective for seasonal affective disorder. These lamps, at a prescribed distance, provide 10,000 lux directed angularly at the user's eyes, without harmful ultraviolet radiation.
Newer research indicates that using a lower intensity of certain wavelengths of light, i.e., the "blue" wavelengths, may be at least as efficacious as using 10,000 lux, at least until one approaches old age, when blue light is no longer more effective than red or green. The most effective wavelengths of blue light are given as ranging between 460 nm and 485 nm by most sources, with some sources specifying peak photopigment sensitivity at 479 nm (in mice).
Only recently have clinical studies been conducted which specifically excluded all patients with any degree of seasonality. Before these studies, there was suspicion that any depressed patients who benefitted from light treatment were really only having the SAD component of their depression treated. However, light therapy is now an established treatment for depression, regardless of seasonality. One advantage it may have compared with drugs is that results may appear more quickly; antidepressant drugs typically take several weeks to reach full effectiveness. Combination of light and medicine has been proven to be more effective and faster than either alone
In the treatment of delayed sleep phase syndrome (DSPS), the timing of light exposure is critical. The light must be provided as soon after spontaneous awakening as possible to achieve the desired effect, as shown by the phase response curve for light in humans. Some users have reported success with lights that turn on shortly before awakening (dawn simulation).
Light energy creates isomerization of the bilirubin and consequently transformation into compounds that the newborn can excrete via urine and stools.
Light therapy is considered a viable treatment for jet lag. Exposure to bright light during the appropriate time periods before, during and after air travel can reduce the symptoms of jet lag and accelerate the recalibration of the body clock. NASA has used timed doses of bright light to prepare astronauts for late night launches since 1991.
There is also evidence that exposure to some frequencies of light (UV in particular) causes the body to release small amounts of endorphins, which would explain the benefit for some disorders such as SAD, as endorphins are often called "the body's own morphine", as well as the concerns for potential tanning addiction, not to be confused with what is commonly called tanorexia, a psychological syndrome wherein patients see themselves as pale, even if they have a substantial tan.
Ultraviolet light causes progressive damage to human skin. 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.
Visible blue light has been suggested to cause DNA breaks, but carcinogenesis has not been demonstrated, and enzymes within the cells are believed to repair the breaks reasonably well. However, cancer has been induced in cells with deliberately damaged repair mechanisms. Also, researchers have questioned whether limiting blue light exposure could reduce the risk of age-related macular degeneration (ARMD).
Modern phototherapy lamps used in the treatment of seasonal affective disorder and delayed sleep-phase syndrome 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.