For what it's worth, and this may not apply to anyone else but me:

On Sunday, I went for a 20 mile ride. The Spring sun is getting brighter, and my new helmet does not have a visor like my old helmet from last year did. I wear prescription eyeglasses. I don't have prescription sunglasses or contacts.
I can do stuff easily without my glasses sometimes, shopping, walking around, etc, no problem unless I have to read small stuff closeup or far away.

So, it was sunny Sunday and I decided I didn't want the bright sun in my eyes while riding. I left my presc.glasses at home and just put on some regular $25 sunglasses (polarized). They seemed comfortable and I wasn't squinting or anything, it felt nice! Halfway through the ride though I started getting a nasty pounding headache right behind my left eye. It got worse and worse, and I decided to take off the sunglasses to see if they were causing it. Within 5 minutes the headache was totally gone. Weird. Not wearing those sunglasses again on my bike.