PhotoperiodEffect.com

21. Red light is somehow okay and other light isn't? That can't be scientific.

Science is full of surprises – the earth moves, invisible creatures cause disease, that sort of thing. In this case the surprise is the existence of ipRGCs, or “dots”. These “independently photosensitive retinal ganglion cells” were only discovered a few years ago.

It turns out our eyes don't just have rods and cones, they have these “dots” too (that don't look all that much like dots) which tell our brain whether it's day or night, and so control melatonin release amongst other things. These ipRGC cells see higher frequency light such as blue or bright green very well, but they are completely insensitive to red light. They don't “see” it at all. So the first light of early sunrise and late sunset is counted as darkness by these cells, and therefore, our brains.

Sometimes the world is just a little more complex than it would be if we personally had designed everything. No doubt it's coincidence that black and white film isn't sensitive to red light either, but for what it's worth, it's a scientific fact that black and white film can't “see” red either, which is why black and white darkrooms are illuminated by red light bulbs.