Moderate heat is something that is extremely beneficial and therapeutic to the human body; after all, if you subscribe to the theories of evolution, we evolved from our ancestors (homo sapiens mark 1, the Neanderthals, homo erectus etc.) who all enjoyed, on average, a much higher air temperature than we endure in the current icehouse Earth (We are just exiting an ice age). If, therefore, we can subject our bodies to regular sessions of moderate heat, this cannot be a bad thing. The question is, how can we do this; how can we heat the body's core rather, than just the periphery, relatively quickly and comfortably.

The answer stems from the fact that both heat and light are energy, just in different forms. Heat is physical vibrational energy of the molecules and light is vibrational energy, but this time vibrations of the electric and magnetic field that is all around us. Fortunately, one can easily transfer to the other if the frequencies of these vibrations match - this is called resonance. It just so happens that water molecules will resonate with light in the infrared, more so in the near and mid. This resonance, that we call absorption, results in water molecules vibrating more, which we in turn feel as heat as the water molecules pass this vibration on to all the other molecules that make up your body.