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Because the Sun feeds plants, is the main cog in the clockwork of the Solar System, and because without it most life on Earth would shrivel up and die, sunlight must have some magical healing properties that science isn't already aware of. The term light woo can be applied to any pseudoscience, magical thinking, or woo beliefs centered around artificially created light or natural sunlight.
The science of sunlight
Through the process of nuclear fusion, the Sun releases radiation across most of the electromagnetic spectrum (except gamma rays) which then gets filtered through the Earth's atmosphere. The ozone layer filters out some of the ultraviolet (UV), but not all of it. Sunlight facilitates the synthesis of vitamin D in most mammals (which occurs from exposure to UV radiation) and fuels photosynthesis in plants. Due to the risk of cancer from prolonged sunlight exposure, the Institute of Medicine doesn't set a recommended minimum level of exposure.
Non-woo light therapy
The Faroese physician Niels Finsen (1860-1904) reported in 1896 that intense light was an effective treatment of lupus vulgaris (tuberculosis infection of the skin), a discovery for which he was awarded the Nobel Prize in Physiology in 1903. His method was later replaced by treatment with anti-tuberculosis drugs. Today it is believed that the anti-bacterial effect did not come from the light itself, but from the irradiation generating highly active oxygen species (similar to peroxide), which are toxic to bacteria.
There is evidence that targeted UVB light exposure is an effective treatment for psoriasis. A common treatment for seasonal affective disorder is "full spectrum" light exposure, although the term "full spectrum" when applied to electrical lighting is a bit of a misnomer, there's some evidence that light exposure performs better than a placebo.
In photodynamic therapy, the patient is given a phototoxic medication, and high-intensity light, usually laser light, is shined on a target area to kill cells there. It is a targeted form of chemotherapy for cancer and various other diseases. Phototoxins are excited into a higher energy state by the light, and transfer the energy to molecular oxygen (O2), which is excited into singlet oxygen. Singlet oxygen is an indiscriminate but short-lived strong oxidizer and is cytotoxic, killing the cells in the target area.
- Sungazing — the New Age belief that staring into the Sun can provide nutritional sustenance.
- Breatharianism — the religious belief that humans can survive entirely on sunlight exposure and air, unsupplemented by food and sometimes even water. In 2013, Naveena Shine posted a video on YouTube stating that humans do not need to eat and can get all of their caloric intake from just sunlight (at least she realized that we need water). She then stated that she was going to "experiment" by not eating for four to six months. After 47 days she ended the fast, claiming that because she was confining herself to her house so that she could film every moment of the fast, she was unable to pay her rent and bills.
- Bates eye method — alternative therapy that attempts to cure defective eyesight or various eyesight related diseases. Features "sunning" whereby the patient exposes their eyes to magnified sunlight.
- Color therapy — alternative therapy that shines differently colored lights on a patient to cure them of various ailments by balancing their "energy".
- Ear light therapy - illumination of the brain through the ear canal, no therapeutic effects but at least we have scientific studies to show that!
- Homeopathic Venus light — one of the stranger homeopathic cures which claims to be light taken from Venus then diluted a shitload.
- Scalar wave laser — a quantum woo laser that supposedly heals and rejuvenates.
- Sonodynamic therapy is a quack pseudomedical treatment sometimes marketed under the name "Sono Photo Dynamic Therapy" [sic], in order to associate it with the (legitimate) photodynamic therapy.
Other light woo
- Lunar effect — the pseudoscientific belief that light from the Moon can turn you into a werewolf or make you crazy.
- Dietary Reference Intakes for Calcium and Vitamin D report by the Institute of Medicine. Published November 2010.
- Møller, et al. How Finsen's light cured lupus vulgaris. Photodermatology, Photoimmunology & Photomedicine. 21(3):118-24. 2005 June.
- Treating psoriasis: light therapy and phototherapy. National Psoriasis Foundation. 2016.
- Fahad Almutawa, et al. Efficacy of localized phototherapy and photodynamic therapy for psoriasis: a systematic review and meta-analysis. Photodermatology, Photoimmunology & Photomedicine. 31: 5–14. 2013 December 20.
- Full-Spectrum Light Sources. Lighting Research Center. March 2005.
- Rosenthal, et al. Seasonal affective disorder. A description of the syndrome and preliminary findings with light therapy. Archives of General Psychiatry. 41(1):72-80. 1984 January.
- Living on Light An Experiment 1 YouTube video, posted 2013 May 3.
- Naveena Shine Will Stop 'Living On Light' After 47 Days Without Food. 2013 June 19.