The reason that you must limit use to no more than 18 hours is because the surface of your cornea needs to have oxygen and saline. Your tears produce normal saline but they do not penetrate the surface of the lens.
Previous contact lenses were made of a type of plastic that was fenestrated - meaning that it had small holes enabling oxygen exchange to take place. Now that these lenses are no longer made, increased wear increases the chance of corneal abrasion, irritation, infection, lack of oxygen to the cornea. Be careful with your eyes - they are irreplacable.