The human body literally glows, albeit in extremely small, barely perceptible quantities. The radiation emitted by a living body is a thousand times less than the human eye is capable of absorbing.

 

The light emitted is not the familiar infrared radiation; it is an emission of actual, theoretically visible light. This light is a by-product of biochemical reactions in the body and body heat and is therefore present in all living creatures.
The glow of the body is an illustrates the production of free radicals in the body, atoms and molecules with single electrons. Electrons usually move in pairs, but free radicals have no counterpart and therefore initiate a series of energetic chemical reactions as they collide with fats and proteins in the cells.

The amount of light emitted by the body varies throughout the day. The human body emits the least light around 10 o'clock in the morning and the most light around 4 o'clock in the afternoon. After that, the light gradually decreases. The process appears to be rhythmic.
The light is mainly emitted by the face, possibly because it is most exposed to sunlight. The skin pigments have fluorescent properties that emphasise the body's low production of light.

It is hoped that in future it will be possible to detect health problems by measuring the light all over the body.