In German mythology the term, 'Ondine' was used as the proper name for a beautiful water nymph who fell deeply in love with a brave knight called Sir Lawrence. The couple married and, when making their vows, Sir Lawrence stated that his every waking breath would be a pledge of his love and faithfulness to Ondine. The couple were very happy and soon Ondine gave birth to a beautiful child. From this moment on however Ondine lost her gift of eternal youth and began to age. As her looks diminished so her husband's interest in her waned and one afternoon Ondine found her husband sleeping in the arms of another woman.
Enraged, Ondine reminded Sir Lawrence of his wedding vows when he said that his every waking breath would be a promise of his love and faithfulness to her. She went on to curse him by saying that as long as he stayed awake he would have the breath of life, but if he should ever have the misfortune to fall asleep then his breath would fail and he would die. Needless to say his life was dramatically shortened.
There is a medical condition called 'Undine's Curse', which is used to describe a rare disorder whereby the automatic control of breathing is lost resulting in the need for every breath to be made consciously. If the person falls asleep then this facility is lost and they die.
From 'Mrs Darley's Pagan Elements' by Carole Carlton