In my opinion. One of the very few things I did not like on my IS was the AUTO rear view mirror level of "insensitivity", in other words, it would not dim neither soon nor dark enough so Dear Mr. Lexus: How about adding an allways on option to the Off and (frequently useless) Auto mode,... or read on.
Again, this is my opinion, perhaps age has made my eyes more sensitive?. Here's what I did. First I tried a little piece of electircal tape to cover the hole beind the mirror, next time you have lights behind bothering you even on AUTO, cover the back with your hand and you'll see the difference. If you place the mirror high, close to the "ceiling" is even worst and if you lower it, blocks my view and won't solve this issue. I don't have tinted windows so can't comment on that.
Another option, the one I will keep from now on: find a leftover pice of tinted paper for windows, all you need is about 1 inch by half inch scrap piece, remove the clear backing, get it wet to activate the adhesive and place it. The improvemnent is noticeable, it won't affect daytime or when ther is no cars behind. The adhesive is very light and wont cause damage if removed.
It increases the sensitivity of the mirror as this guy seems to work by contrasting the lights in front with the ones behind. Not necessarily the right approach since lights behind can bother a lot even if you are on a well lit street or at dusk.
If sometimes you are bothered by lights behind you, try it.