My rear view mirror does this, I wish my side mirrors did too. Although recently I've noticed some cars headlights can even pierce my rear view mirror's polarized dimming. It never used to be a problem in the past. I've seen the difference when drivers turn their high beams on and off. It always did a great job against driver's brights including large trucks. But occasionally there's now a vehicle with the light of a thousand suns that is too bright for the auto-dimming.