In order to understand the dangers of overnight wear, we must understand how the contact lens interacts with the eye.
In normal wear, the contact lens does not sit directly on the cornea. It floats on the tear film and moves up and down a fraction on every blink. I won't go into the all the reasons for this at this time, but it is important. Also, on every blink, the eyelids bring down a fresh layer of liquid to re-wet the cornea and contact lens. When sleeping, this interaction does not happen. Without lens movement and/or re-wetting, the contact lens will dehydrate and stick to the surface of the cornea (especially if alcohol is involved). If this happens, and you rub an eye in your sleep too hard, the lens will take patches of epithelial tissue off when it does move. And because the cornea has no blood vessels to bring anti-bodies etc. to the site, it is extremely easy to get a bacterial infection. Once an infection sets in, the eye can be lost with 24 to 48 hours.