The spectrum of colors that our eyes are capable of processing is although you do not believe very limited, so limited that we are only able to perceive some spectra of referential lights, leaving us unable to see the world for what it really is. However, what happens when the spectrum of light that our eyes can process is compromised, or rather, what do we see when we see nothing? Probably the general answer is that we see nothing, but that's not correct, let's remember that our eyes don't go off and on every time we close our eyelids, that is to say, that seeing nothing is almost impossible. So we can discard that answer, the next thing that will occur is that, well if we analyze a little what we see is the inside of our closed eyelids, but if that were so, why when we close our eyes we do not see the color of the inside of our eyelids? So that certainly is not the correct answer either. As much as our vision is affected by something that prevents it from perceiving correctly what is in its field of vision, our eyes will always be able to perceive something and when not, our mind will be responsible for filling the gaps.
The truth is that the thing is a little more complicated when we close our eyes we see something that is nothing, nor is it dark and that certainly is not the inside of our eyelids, but something else. Well, in this post I will try to explain to you the best I can what it is that we "see" when our eyes are closed.
What we see (I know it does not make sense) when we close our eyes is something that closely resembles the black color or the so-called darkness, but in no way is not one or the other, but it is something totally different than It has been called Eigengrau.
Eigengrau
Eigengrau (German: "intrinsic gray", lit. "own gray";), also called Eigenlicht (Dutch and German: "own light"), dark light, or brain gray, is the uniform dark gray background that many people report seeing in the absence of light.
Wikipedia
Basically what we see, rather what our mind makes us believe we are seeing is a void with a dark color that closely resembles a kind of opaque gray. Many at this time will be closing their eyes and saying but what are you talking about? I do not see that opaque gray of which you speak, well the answer to that is that in general conditions when we close our eyes the light continues to fall on our retinas and we can see some flashes of colors that we would see with our eyes open, but when the light is very low and we close our eyes what we see is the Eigengrau.
To make it shorter when we close our eyes and the light does not sneak into our retinas what we see is the opaque gray called Eigengrau.
The Eigengrau is a kind of mirage generated by our mind in conjunction with the retina to fill the void left by being with closed eyes and is that after everything something has to be there to cover the absence of light. Because Eigengrau is both a biological and a psychological phenomenon, it has been determined that not all people perceive it in the same way and that there are many variations of the "Opaque Gray", hence each person can have their own appreciation of the Eigengrau.
From the biological point of view.
There are many types of research that have been made to try to understand why Eigengrau originates after all, it would be normal to see everything black or inside our eyelids as I said at the beginning, but the reality is that our body never stops unless it is strictly necessary or that some organ or part of the body is damaged and fails. That is why even when we close our eyes they continue to work so that in many occasions the light filters through our eyelids and we are able to see residual light, because even when the light does not cross our eyelids, the part of our brain responsible for processing the light that passes through our retina will continue to work and what they will do is to recompose and recreate in a certain way the light that they had already captured in order to fill the gaps that have remained after closing their eyes.
Basically, this means that after closing the eyes both the retina and the nerves responsible for the vision will continue to work and create the Eigengrau to occupy the place that the absence of light has left. That is why I was referring to the Eigengrau as a kind of mirage.
However, it should be noted that this may not be the only cause of Eigengrau, as this could be the result of the worsening of our perception of movement and sounds, resulting in our mind making some kind of perception, in addition to the neuromelanin discharges that originates when we are in the absence of light, meaning that this pigment is derived as a result of us depriving ourselves of perceiving light, although in this case, other factors would intervene, such as the oxidation of dopamine that is essential for the creation of neuromelanin.
From the psychological point of view
To say that the Eigengrau is a hallucination may be something strong, but also the most viable explanation since from the biological point of view finding a solid explanation for such a phenomenon can be complicated, in fact I say to you that this phenomenon could be seen as a kind of mirage and is that after reading several sources of information I still think that is a mirage.
I get involved in the word mirage because to call it hallucination I just do not like it, however, it must be emphasized that the eigengrau is something generated by our mind, either from remnants of light in our photoreceptors or because of a photosensitive pigment or any another explanation that you decide to believe is valid.
The eigengrau besides being something that has been created by our mind to fill the void left by the absence of light, is a type of hallucination that usually appears at moments where our mind is restless, I say this because to many people close their eyes is perhaps synonymous and signal to fall asleep, but in many cases especially when insomnia makes its appearance and we close our eyes for a long time our brain begins to elaborate colors that are usually associated with dull gray and with black everything as a sign that at some point in our brain something is failing and that something is keeping us awake.
In short, the eigengrau is often a hallucination that occurs when we are facing a disturbance that prevents us from sleeping, but which nevertheless is a hallucination that in a certain way has no consequence since it is only made to fill a void.