I wore 'high permeability' contacts every day for many years and tended to leave them in for a few nights at a time when sleeping outdoors. I just used eyedrops and never had a problem. Then one day I started to feel like I had something in my eye, all the time.
The cornea, explained the eye doctor, is supposed to get its oxygen directly from the air. Even the most permeable lenses in the world will interfere to some extent. Corneal neovascularization is what happens when oxygen deprivation occurs too often or for too long and the body responds by growing brand new blood vessels right into the cornea itself. These new vessels are very uncomfortable under a contact lens (which will damage and scar them), and they also mess up night vision, because when the pupil dilates these blood vessels are in the way. The good news is that they will eventually become redundant and go dormant if oxygen is restored. The bad news is that they ONLY go dormant, and they'll be quick to come back if the need arises.
I had never heard of corneal neovascularization, I always thought hygiene and infection were the big concerns with contacts. According to this doctor, we have a lifetime quota of contact lens wear, and the quicker we use it up the sooner we have to quit. After ten months, I can now wear them again — but not for as long and not as often, certainly not overnight. Or I'll feel like I have something in my eye.
So, um, yeah, watch out for that.

