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.