Falling asleep without taking out and rinsing contact lenses is a bad habit.
Poor lens hygiene will cause eye ailments including microbial keratitis, an infection of the cornea, the clear frontal part of the eye where lenses sit. If it is untreated in time, it can lead to permanent visual damage, and in extreme cases, blindness.
Sleeping in contact will cause red and irritating eyes. As well as sensitivity to light and pain, other symptoms include redness, discharge or watering and blurred vision. All of these will get worse until the appropriate antibiotics are administered.