Anyone who's ever struck a match in a dark room (like Jason Gaines, here) knows that the fire at the match tip emits visible light and we do not. Because 98 °F is a lot cooler than the sun, our bodies radiate light at much longer wavelengths than visible light. We radiate at infrared wavelengths, at about 9m — almost 20 times longer than the wavelength of visible light. It's just that our eyes are not sensitive to wavelengths that long.