Living things have been known to glow thanks to a nifty trick called bioluminescence, but it’s something we more typically associate with animals of the deep sea than those stomping around surface-side. It may surprise you, then, to learn that bioluminescence has been detected in humans. That’s right, we glow in the dark – it’s just really, really, really faint.

That was the discovery made by researchers in a 2009 study who used incredibly sensitive cameras to effectively watch naked people sleep. Bit creepy, sure, but it also shone a light on the light that we unknowingly emit.

"The researchers noted that 'the human body actually emits a subtle glow.' As for the reason we can't perceive it? 'The brightness of the light released by the body is a thousand times dimmer than what our unaided eyes can detect.'"

The shimmering phenomenon was noticed in five men in their twenties who were placed under standard light-dark settings and encouraged to take short naps in front of a cryogenic charge-coupled device (CCD) camera capable of sensing light down to the level of a single photon. The researchers pointed out that the camera needed to function at a temperature of –120 °C (-184 °F), but thankfully, the participants did not have to endure such extreme conditions.

They were, however, routinely sampled for saliva to measure cortisol levels, and had their surface and oral temperature checked before and after photon measurements were taken. Cortisol is a biomarker of endogenous circadian rhythms, which the researchers wanted to have tracked so that they could compare it against any changes observed on the camera.

Interestingly, our natural radiance appears to fluctuate throughout the day, peaking in our facial glow. The driving force behind this variation is probably linked to our circadian rhythms.

Chronobiology is a branch of science that studies cyclical physiological phenomena, and it’s established that the circadian clock is the main regulator of metabolism. We see it in the way we burn up glucose and consume oxygen, both of which – the researchers say – show robust rhythms in the main mammalian circadian center.

The mitochondria, often referred to as the "powerhouse" of the cell, generate energy essential for our survival, but in the process, they also release small quantities of reactive oxygen species (ROS) as byproducts. These ROS engage with various molecules such as proteins, lipids, and fluorophores, resulting in excited states that emit biophotons. This phenomenon is what causes "the human body to shimmer in harmony with the circadian clock."

Oh, look at us, all dressed up!

Bioluminescence relies on enzymatic activity to glow, but there’s also another way that living things can glow and we’re increasingly finding it in more and more species.