Another from the "I've Never Heard This Before, But It's Great" file, here's John Sheppard's "In pace in idipsum." (The way it alternates between chant and polyphony is really wonderful.)
In pace, in idipsum dormiam et requiescam.
Si dedero somnum oculis meis,
et palpebris meis dormitationem,
dormiam et requiescam.
Gloria Patri, et Filio, et Spiritui Sancto.
In peace and into the same I shall sleep and rest.
If I give slumber to my eyes
and to my eyelids drowsiness,
I shall sleep and rest.
Glory to the Father, and to the Son, and to the Holy Spirit.
BONUS: Here's another version, also listed as "In pace in idipsum" and also claiming John Sheppard as its composer. And I think I might like it even better.