What Is Infrared Heat and How Does It Affect Your Body? Infrared light sits just beyond the visible red end of the spectrum. It's a form of light you can't see, but you can feel it as warmth. It's divided into three categories based on wavelength - near-infrared, mid-infrared, and far-infrared. Each behaves differently in the body, and those differences matter when it comes to how infrared is used in training and recovery.