An experiment in complete darkness
Five healthy men sat in a light-tight room and were photographed every three hours by a camera with a heavily cooled CCD sensor. Cooling reduces noise, so the sensor can pick up even individual photons. The images showed that the face glowed the most, especially the cheeks and forehead, and that the brightness rose and fell regularly over the course of the day.
Why we glow
Light arises from chemical reactions in cells. Burning nutrients produces reactive oxygen species, which react with fats and proteins. Some of the resulting molecules shed their excess energy by emitting a photon. Scientists call this ultraweak photon emission. Experiments in 2025 showed that live mice emit more such photons than dead ones.
Light that a thermal camera can see
We radiate far more energy as heat. A body at a temperature of around 37°C glows mainly in the infrared range, at a wavelength of around 10 micrometres. The eye can't see it, but a thermal camera can, which is why it can find a person in the dark or in smoke. Security cameras with night vision mostly use a different trick: they light themselves up with infrared LEDs.
The link to technology
Capturing such faint light was made possible by CCD sensors. They rely on the photoelectric effect explained by Albert Einstein, and the first person to use one in a digital camera was Steven Sasson. You have a similar sensor, just far less sensitive, in the camera in your phone.