Certainly there are consequences of overnight contact lens wear, especially lenses that were never intended to be worn overnight. Traditional soft or hydrogel contact lenses generally have a high water content which makes them quite comfortable. This material though does not have a high Dk/T which means very little oxygen will arrive at the cornea. The cornea is avascular tissue, and it is that way for a reason, so that you can see through it. Without blood vessels though much of the oxygen the cornea receives must come directly from the air. Depriving the cornea of oxygen while wearing contacts is exacerbated at night when your eyes are closed. The Na/K ATPases of the corneal endothelium dont work very well in low oxygen conditions and as a result your cornea will swell overnight. In addition there is also evidence that contact lens wear has an effect on corneal endothelium which does not tend to regenerate itself (there is still debate about this as stem cells have been found in the corneal endothelium). Also in relatively extreme cases the hypoxia can lead to corneal neovascularization.
It may be surprising to hear that hard contacts also known as rigid gas permeable lenses have a much greater Dk/T than hydrogel contacts. I
n the relative recent past a new material has been developed for soft contacts known as silicon hydrogel. The silicon has a high Dk/T which means less corneal swelling, so much so that some of these lenses are even approved for 30 days of continuous wear. If you intend to sleep in your contact lenses at all I would make sure that you tell your OD or OMD so that they can Rx you silicon hydrogels. Lenses made of this new material include Focus night and day, and o2 optics as well as Acuvue Advance and Acuvue Oasis among others.
There are inherent risks of wearing lenses overnight, even with the new silicon hydrogels the risk of contact lens related infectious keratitis is increased. As always you should go to a health care provider to seek advice. Always follow their wearing and lens care regimen. As we always say contact lens wearers tend to be the addicts of the eye care world. People dont tend to take good care of their eyes reusing cleaning solutions if they clean their contacts at all, not removing contacts at night etc..etc..etc
and everything is fine and dandy until they end up with a corneal ulcer, and thats no fun for anybody.