Selene fell in love with a shepherd and asked Zeus to keep him young. He granted it by putting Endymion into a sleep he never wakes from, and she visits every night.

The Greeks were not entirely comfortable with the story and told it several ways, some of which give the couple fifty daughters, one for each lunar month of an Olympiad. It is the origin of 'Endymion sleep' as a phrase, and Keats got a very long poem out of it.