Why you shouldn't sleep with contact lenses on
Oxygen from the air is vital for the health of the cornea (the front clear surface of the eye). Whilst modern soft contact lenses allow sufficient oxygen to pass through the lens in daily wear, when you sleep your eyes are closed thus reducing the available oxygen and potentially causing problems for your eyes (unless using specific lenses designed for extended wear).
Falling asleep with your contact lenses on can, therefore, increase the risk of complications such as developing a corneal ulcer, which can be very painful. Despite being a very rare and often treatable condition, it often causes scarring, which may have long-term effects on your vision.