Many contact lenses are made to be slept in. I know it can be more convenient to sleep in contacts rather than taking them out at night and putting them in the next day.
In my experience, the patients I see in my office with contact lens related problems like corneal ulcers or keratitis are those who sleep in their lenses.
While these conditions typically respond well to treatment, it is quite possible that they can leave permanent damage, like a corneal scar, that will affect vision for the rest of your life.