The prayer of sleep instituted, according to the canonical hours, to commemorate the laying down of the body of Jesus Christ in the tomb. In churches and monasteries compline is said after vespers and before the raising of incense for the evening service. The DIDASCALIA (Qeladah, 1979) and Ibn al-‘Assal’s Kitab al-Qawanin (Book of Canon Law) include compline among the seven canonical prayers.
It appears that compline was initially confined to the Egyptian church, as Saint BASIL THE GREAT does not include it in his earlier text of prayers. Instead, he divided the sext (the sixth hour) into two parts: the first preceding and the second following the evening meal, to bring the total number of prayers to seven.
Later, however, Saint Basil chose to add compline after vespers, incorporating with it “At midnight I will rise to give thanks unto Thee because of Thy righteous judgment.”