Rods and Cones are Not Alone

There are approximately 120 million rods and 6 million cones in the retina which help us to see, but did you know there is another type of light sensitive cell there? Intrinsically photoreceptive retinal ganglion cells, or ipRGCs for short, are a type of cell in our retina that has nothing to do with our vision but it does sense light. It is particularly sensitive to blue wavelength light like that of the bright blue sky but it can also be stimulated by sources of blue light not found in nature (like that emitted by our smart phones, tablets, computer screens and TVs).

In the 1990s Czeisler et al found that these ipRGCs are present and functioning in people with certain types of blindness. So even though these people could not see or perceive the presence of light, their retina still sensed it thanks to their ipRGCs and their circadian rhythm was set by it.

Science hidden in plain sight, indeed.