I've answered this before on the forum but don't remember where.
Basically, most Daily Disposables have low oxygen permeability (Dk 18-30 say). Modern silicone hydrogel lenses tend to be Dk 100-140. The J&J 1 Day Acuvue TruEye is a recent exception to the Daily Disposable lenses.
The cornea has no blood vessels, but is living tissue so must be fed with oxygen somehow. Normally this is directly from the air in front of the cornea and the tear film, which carries some oxygen. If you wear a
contact lens, then it is more difficult for oxygen to flow to the cornea through the lens and proportionately more oxygen must be supplied via the tear film. So, the contact lens must fit properly for the tear film to pump properly. Even so, it is likely with very low Dk lenses, the eye will not have enough oxygen.
Some people complain that the Daily Disposable lenses become dry and difficult to remove if worn too long. If it is dry, the tear film will also be compromised, which will hinder oxygen flow to the cornea via the tear pump.
At night, the eyelid is closed and even without
contact lenses, the eye suffers a deficiency of oxygen. It does receive oxygen from the capillaries in the back of the eye lid, but not quite enough and overnight the eye will swell to some extent. With low Dk lenses, this lack of oxygen supply to the eye overnight is much more extreme as the lenses act as a barrier to oxygen flow.
The eye will compensate for a lack of oxygen to the cornea by creating new capillaries which grow into the cornea to supply oxygen there. Trouble is that the cornea needs to be clear and free of capillaries to give sharp clear vision. This capillary growth is called neovascularisation. Even if you switch to a higher Dk lens after developing this neovascularisation, the capillaries will drain of blood but will remain as ghost capillaries. These don't seem to go away.
In addition, wearing lenses overnight or 24/7, even if your eye care professional agrees, is not the most healthy thing for your eyes. It increases the risk of corneal infections & ulcers (even if the lenses are silicone hydrogel) and worse if you are also a smoker.
knotlob