I have only had this experience once and it was totally unexpected, but the change was dramatic. When it happened, I had my eyes shut and thought I was just imagining this brightness, so I opened my eyes and the brightness was still there. It didn’t seem to matter if my eyes were closed or opened as I was seeing the same thing. Light was filling the room and I was aware I was not “seeing” with my eyes, but was experiencing light within my whole being.
This technique of being immersed in total darkness and then having the experience as if a switch has happened, and then suddenly being filled with total radiant light was a method used by ancient cultures for spiritual awareness and communication with those on the astral plane. Cultures, such as the ancient Native Americans who would journey to Chaco Canyon for spiritual celebrations for the solstice would engage in these types of spiritual activities. They would go into the pitch-black dark windowless chambers and in those rooms, would experience moments of radiant light from the astral connecting to the physical plane.
The experience of being in extreme darkness and then suddenly being immersed in extreme white light from the astral can be a profound and memorable experience.