Darkness is thoughtlessness
Your inner space needs a kind of silence once in a while. Utter silence relaxing into the deep darkness, causal body. But the fear of darkness will even reduce the quality of sleep. All fears are directly or indirectly related to this fear of darkness. Depression is outcome of your misunderstanding and fear of darkness. When you close your eyes, if you notice movement of colors or movement of light streaks then they are thoughts. Darkness is the base and everything else that is projected in this world or even in your dream world is the play of light and shadow or light and slide. Everything is light in different frequency and wavelengths. Even thoughts are your thought shafts in form of light with its own wavelength. When they become too much, you will experience them as a constant inner chatter or verbalization. Color of thoughtlessness is darkness. Color of no mind is darkness.
The reason for the sharpness in my eyes is because I lived in a place where there was no artificial light. I lived more than nine months without seeing any manmade light in the forest. My eyes got completely healed and returned back to nature, it's own nature. This technique is about bringing back your body back to nature. 80% of connection with world happens through our eyes. If your eyes are brought back to nature, 80% of your being will be brought back to nature. You will be connected to your roots.
I saw when I lived without lights, not only was I able to see in darkness, the number of thoughts has drastically reduced. That is one important thing. Try this very powerful technique. Lie down in bed and switch off light. Let it be utterly dark. Don't fall asleep. You have to remain awake in darkness. Of course, in cities you can never have the complete darkness. Your eyes have to be open and it should be completely dark.