Even with contacts, having sunglasses that allow you to see when you move from sun to shade remains an issue. My best bike purchase this summer was a pair of Tifosi sunglasses that not only are photosensitive, but go almost clear in low light situations.

For the record, I'm blind as a bat and wear gas permeable sunglasses. I've never tried riding with my glasses on and I imagine I'd have peripheral vision issues. I just see much better with my contacts. I lost one on a ride last year, but I've otherwise had few issues with them. I absolutely must have sunglasses on when I ride though. Dirt, debris and the like are a no no with the gas perms.

To be honest, I don't know if taking your sunglasses off when you come into a shady area is the easiest solution. At least in my neck of the woods, shade is synonymous with bad drainage and potholes. Both hands have to be on the handlebars.....