Diana (Artemis) is the goddess associated with the moon, and Endymion was the moon's lover. The moon asked Zeus to grant Endymion eternal youth so that he could be her lover forever; to accomplish this, Zeus put Endymion into an everlasting sleep so that the moon (or Artemis or Diana) could visit him every night as she traveled overhead in the sky. You may know the opening lines of Keats' famous poem, Endymion:
A thing of beauty is a joy for ever;
Its loveliness increases; it will never
Pass into nothingness; but still will keep
A bower quiet for us, and a sleep
Full of sweet dreams, and health, and quiet breathing.