Infrared light is part of the Sun's invisible spectrum. One of far infrared light's characteristics is the ability to easily penetrate human tissue.

Remember the last time you were sunbathing with sunscreen? Were you still hot? Of course you were because the sunscreen only stops the UV (Ultraviolet) light from hitting your skin. The far infrared heat (Far infrared light waves) penetrates your skin, giving you that wonderful natural warmth.