What's the deal with the polarized windshield?

Not sure if polarized is the right term but I don't know what it is and I find it annoying. When night driving, the upper part of the windshield at about eye level is darkly shaded and the division line moves up and down with my head if I try to look over or under it. Is this to decrease glare from oncoming traffic? If so, I haven't noticed enough benefit from that to outweigh having half of my windshield dark making it harder to see out of it at night.

 

Do you wear glasses? It may be messing with your vision through the glass.

 

No glasses. A lot of stuff has changed since the 1997 Taco I just got rid of and it's all taking some getting used to.