Lighten our darkness, we beseech thee, O Lord;
and by thy great mercy defend us
from all perils and dangers of this night;
for the love of thy only Son, our Saviour Jesus Christ. Amen.
This brief but lovely prayer is found in the 1559 version of The Book of Common Prayer, and had its antecedent in pre-Reformation English Catholic use.* The prayer was intended to be said at Evensong, the last Hour of the day.
The redoubtable Father Hunwicke has an exposition of this prayer for us, which is most interesting.
Whatever the intention of the ancient author of this oration, we can now look at Cranmer's choice of the translation "lighten" in two ways :
"Shed your light upon our darkness" or
"Lift the burden of our darkness".
In such an understanding, Darkness may refer to our sinfulness, or to our spiritual or intellectual blindness. It is certainly a prayer for those who wish to be at rights with God before sleep descends.
* UPDATE A priest-reader has kindly written to me to explain that this prayer was of Catholic origin, being the last prayer of the Office of Compline in the Sarum Use.