A respiratory disorder, also called congenital central hypoventilation syndrome (CCHS) or primary alveolar hypoventilation, which is fatal if untreated. Because of a congenital failure of the body's involuntary control of breathing, people with the disorder must take each breath consciously; they typically die during sleep.
In German mythology, Ondine was a water nymph who fell in love with and married a mortal man, Sir Lawrence. He swore to her that his every waking breath shall be his pledge of love and faithfulness to her. Years later, she caught him in the arms of another woman. She cursed him thusly: "You swore faithfulness to me with every waking breath, and I accepted your oath. So be it. As long as you are awake, you shall have your breath, but should you ever fall asleep, then that breath will be taken from you and you will die!"
Sir Lawrence stayed awake as long as he could but eventually fell asleep from sheer exhaustion, and he stopped breathing and died.
Ondine's curse gives new meaning to the phrase, "I'll sleep when I'm dead."