Far infrared light (not be confused with ultraviolet light which causes sunburn) is thermal energy naturally produced by the sun. The name means "below red" and comes from the Latin infra, "below", red being the color of visible light with the longest wavelength. It is the warmth you feel penetrate your skin when you are outside in the sun. At the molecular level, Far Infrared light exerts strong rotational and vibrational effects that are biologically beneficial. Far Infrared Thermal Rays elevate the heart rate and improve blood circulation without elevating blood pressure. When penetrating the skin, far infra-red causes a thermal reaction which elevates tissue temperatures. The human body then reacts by dilating all the blood vessels.

An excerpt from Alternative Medicine: The Definitive Guide (by Burton Goldberg, John W. Anderson, & Larry Trivieri), pg 670:
'Among FIR's healing benefits is its ability to stimulate inflammation, which is necessary for a period of time to heal injuries (such as a pulled muscle). Far Infrared light is also capable of enhancing white blood cell function, thereby increasing immune response and eliminating foreign pathogens and cellular waste products in the body. Additional benefits of FIR include the ability to stimulate the hypothalamus, which controls the production of neurochemicals, which is involved in such biological processes as sleep, mood, pain sensations, and blood pressure; enhancing the delivery of oxygen and nutrients to the body's soft tissue areas; and the removing accumulated toxins by improving lymph circulation.'