Naturally, I come across this when I'm away from work for a few days and have no access to research I did on this topic. There are studies, but they aren't great -- definitions for 'sun involved' are broad enough you can't actually claim the sun was involved or not and there aren't any controls.

Numbers are from memory, but what I recall from a few papers which dove deeper into it was if the sun is within 25 degrees of what you're looking at (the study came from tunnel exit design in Spain, you don't want a tunnel exit pointing at the sun) you can have issues with sun glare. This angle increases with age.

If I remember I can try and dig up some cites in a few days.