With one's eyes open, the brain craves visual input and tries to make sense of the nothingness. With my eyes open in the dark I can "see" my hands and arms (years of open tray sheet film development) -- my brain creates the image based on muscle memory perhaps (based on the tightness/looseness of all the arm and hand muscles, the hands must be positioned like thus...). I use to worry that there must be some light leaking into the room, but there was none, and I could see no sign of other objects, just my hands and arms (and no detail -- more of a sense of where they are). But if I close my eyes, I can no longer "see" them! Perhaps as Clayne said -- it is like turning off the radio, and my brain no longer has to create an image out of the darkness.
If I find myself straining my eyes in the dark, I close them.
QG -- I developed 3 rolls of 120 last night -- seeing the glow is fun...and I repeated the old test -- pulled one slowly and one quickly. The quick one (nice bright glow) fogged the film locally, and the slow one (no glow) did not. Fun!