Office of Readings, this hymn attributed to Pope St. Gregory the Great. I also found a little more information about this particular hymn at Thesaurus Precum Latinarum, a neat little website that you may want to bookmark. There, Michael Martin writes,
This traditional Matins hymn is used in the Liturgia Horarum for the Office of the Readings for Wednesdays of the 1st and 3rd weeks of the Psalter during Ordinary Time. Likewise it is found as the Matins hymn for Wednesdays in the Roman Breviary.
I get a tingle up my spine when I hear words like Matins. Formerly the early morning prayer of the Church, it has now been replaced by the Office of Readings in the modern form of the LOTH. But I like the original Latin usage to signify the various prayers of the day, such as Lauds, Vespers and Compline. Some very neat vocabulary words to help orient your day around Our Lord.
More trivia from Michael's website: this English translation of this hymn is by Blessed John Henry Cardinal Newman.
Rerum Creator optime (Thou Madest All)
Thou madest all and dost control,
Lord, with Thy touch divine,
cast out the slumbers of the soul,
the rest that is not Thine.
Look down, Eternal Holiness,
and wash the sins away,
of those, who, rising to confess,
outstrip the lingering day.
Our hearts and hands by night, O Lord,
we lift them in our need;
as holy Psalmists give the word,
and holy Paul the deed.
Each sin to Thee of years gone by,
each hidden stain lies bare;
we shrink not from Thine awful eye,
but pray that Thou wouldst spare.
Grant this, O Father, Only Son
and Spirit, God of grace,
to whom all worship shall be done
in every time and place.