The history of light therapy dates back more than 3000 years in India, where sunlight was used for therapeutic purposes and recorded in the sacred Hindu texts. Today, the beneficial effects of light therapy are well-established and they involve the use of visible light or non-visible ultraviolet light to treat a myriad of conditions.

Conventional light sources include incandescent lamps, fluorescent lamps and electric ares. Light therapy boxes that emanate full-spectrum light similar in composition to sunlight are also often employed. Identifying the individual circadian rhythm and selection of proper timing is critical for adequate administration of light therapy.

The rationale behind light therapy

Two inherent properties define the biologic grounds of light therapy (also known as phototherapy) – wavelength and intensity. The wavelength range of visible light is roughly from 780 nm (red end) to 400 nm (violet end). The circadian rhythm is often referred to as body clock and it represents a 24-hour cycle that regulates sleep and other physiological processes.

In the past, researchers most often utilized bright white light (i.e. a mixed spectrum of wavelengths that imitate daylight) in order to study light effects on human circadian rhythm. On the other hand, recent studies have demonstrated better effectiveness of short wavelength blue light (approximately 460 nm) with better phase shifting properties when compared to the rest of the visible light spectrum.

Light therapy targets the subjective human biologic clock and attempts to reset the phase of the clock's activity, relative to the cycles of light and dark. Thus, common goals of light therapy are to synchronize the sleep-wake cycle with the subjective night, to facilitate sleep at a desired time of day or night, and to attain indirect effects on the mood a patient.

Photodynamic therapy (or PDT) is a special kind of light therapy that uses drugs called photosensitizing agents along with the light in order to kill cancer cells. It can also aid traditional light therapy in shrinking sebaceous glands. The drugs only work after they have been activated by certain kinds of light, and in the medical literature the procedure is also known as phototherapy, photoradiation therapy or photochemotherapy.

Application in clinical practice

In psychiatry and neurology, light therapy has been extensively used to treat a wide range of disorders that are usually classified in three broad categories. Those include disorders caused by desynchronization between the body's circadian rhythm and the external environment, mood disorders and disorders which contain elements of both.

Phototherapy with visible light (specifically blue light) has been shown to improve skin condition in cases of acne and blemishes. Upon reaching the sebaceous glands in the skin, blue light successfully excites porphyrins, which are compounds produced by bacteria that cause acne (namely of the genus Propionibacterium). When they are activated by light, porphyrins can destroy the bacteria from the inside out.

Neonatal jaundice represents the yellowing of the skin as a result of increased levels of total bilirubin in infants. Blue-light phototherapy is the treatment of choice for otherwise healthy newborn children with elevated unconjugated bilirubin.

Photodynamic therapy has played a significant role in the treatment of dermatological diseases such as actinic keratinosis, various cutaneous lesions and non-melanoma skin cancer. Furthermore, applications of this type of treatment have been used in cases of periodontal diseases, oral leukoplakia, oral cancer, macular degeneration and a panoply of solid tumors from diverse tissues and organ systems.

Low-intensity light therapy (LILT) is becoming a promising tool in the treatment of a wide variety of conditions, such as arthritic pain, delayed wound healing and acute stroke. Improvements in these conditions have been attributed to rapid elevation of ATP, increased angiogenesis and anti-apoptotic activity, as well as a surge in heat shock proteins and total antioxidants.