Endymion was considered the personification of creeping sleep , genius of the night, the personification of death. 50 daughters of Endymion were identified with 50 lunar months is a four-year gap between Olympics . Later commentators believed Endymion first meteorologist , studied for 30 years, the changes of the moon ( Selene ), which is indicated in the myths of the Thirty Years' love for Endymion Selene and his thirty-year dream. The expression " sleep of Endymion " proverbial as synonymous with long sleep .