I don't know if this is only happens to me, but if I am in a well lit room and I stare at one point or just look at one area without moving my eyes around my vision in that specific light setting becomes dimmer. I can look around and everything feels like it is dim and lacks the bright light that originally radiated through the room. So I was wondering how and why this happens? I suspect(I am quite dusty in my ophthalmology) my eyes maybe relaxing and taking less notice of the intensity of the light, if so what makes it do so? As a side note this occurrence hasn't affected my life, I am just curious as to why it happens. Another note response to some of the comments, this isn't like walking into a well lit room and get bombarded by the light and eventually adjusting to it. This is like sitting in a well lit room for 15 minutes and then things just become dimmer and less intense. I've also observed this only happens with artificial light and may have some link to the type of light bulb.
I would like to add that this happens if I concentrate on one area or object. So if I were to read a book or be on my laptop, the light around me would feel dimmer. I suspect it has something to do with my eyes focusing on one source, if I were reading a book and this happens, the words become more prominent.