Full-spectrum light is light that covers the electromagnetic spectrum from infrared through near-ultraviolet, or all wavelengths that are useful to plant or animal life; in particular, sunlight is considered full spectrum, even though the solar spectral distribution reaching Earth changes with time of day, latitude, and atmospheric conditions. ‘Full-spectrum’ is not a technical term when applied to an electrical light bulb but rather a marketing term implying that the product emulates natural light.
Some products marketed as ‘full-spectrum’ may produce light throughout the entire spectrum, but not with an even spectral distribution, and may not even differ substantially from lights not marketed as ‘full-spectrum.’
Full-spectrum fluorescent lamps are used in the art studio by artists who paint pictures on canvas when they paint at night or inside (ideally, during the day the art studio should have north sunlight, but many artists don’t have access to north sunlight so they use full-spectrum lamps instead) in order to make sure that the colors they are using appear in their natural hue as they will appear when the painting is displayed in a home or in an art gallery.
Full-spectrum lamps are also used by color scientists or color matchers in paint stores to match colors at night or inside when they don’t have access to north sunlight. Gardening under lights keeps plants blooming almost year-round, for a wintertime harvest. Some plants grow better when given more of a certain color light, due to the mechanism of photosynthesis. In recent years, full-spectrum lighting has been used in the treatment of seasonal affective disorder (SAD) through the use of ‘light boxes’ that mimic natural sunlight, which may not be available in some areas during the winter months. Light is an environmental stimulus for regulating circadian cycles.
Lightbox therapy, otherwise known as phototherapy, is a recognized modality for depression (such as SAD). It is also the primary treatment for circadian rhythm sleep disorders. Depending on the quality of the light, it is estimated that 10,000 lux is needed for effective treatment. Not all light boxes are the same, and some produce only blue or green light. Several independent investigations of light therapy have cast doubt on its efficacy.
The Daily Omnivore 
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