The closing Office of the day, held after sunset, when night begins, seems to be of monastic origin, though eventually it became part of the Roman Office as well. Compline is mentioned by Saint Basil the Great, who died in 379 A.D., and also by Saint John Chrysostom, who died in 407 A.D.. Traditionally the same three psalms were prayed each night: 4, 90 and 133. Hence, the psalms could be prayed by heart, often in the dark. These three psalms contain clear references to the night, going to rest, dwelling in the shelter of the Most High, protection of the angels, etc., and so are perfect for the end of the day.