Ptosis is a condition in which your upper eyelid droops, sags or falls over your eye. It usually happens because your levator muscle — the muscle that lifts your eyelid — doesn’t work as it should. The condition can limit your vision or block it completely, depending on how much your lid droops. Ptosis of the eyelid can affect one or both upper eyelids. It can also be worse in one eye than the other.