This is a 10th-century Matins hymn translated from Latin by Percy Dearmer, the author of ‘Draw us in the Spirit’s tether,’ who collaborated with Vaughan Williams to edit the original English Hymnal just over a hundred years ago. The music is not quite as old; it is a 17th-century French church melody, roughly of the same vintage as the tune for ‘O come, O come Emmanuel.’
In 10th-century monastic practice, Matins was not morning prayer; it was middle-of-the-night prayer, sung well before sunrise as the first office of the day. In fact that Latin of the very first line, nocte surgentes (“rising at night”) has been rendered in English as “now the night is over” to bring it in line with more contemporary understanding of just what constitutes morning.
It is of passing interest to speculate on the significance behind why a particular hymn is selected as the first in a collection. Most Methodist hymnals are prefaced with John Wesley’s “Directions for Singing” followed by his brother Charles’s ‘O for a thousand tongues to sing’ as the first hymn. Traditional Lutheran hymnals (the German ones more so than the American) are arranged according to the seasons of the church year, beginning with the Advent hymn “Savior of the nations, come” (Nun komm, der Heiden Heiland, translated by Luther from St. Ambrose’s Veni, redemptor genitum).
In The Hymnal 1982, I think that beginning with the morning hymns is an acknowledgement of how important the office of Morning Prayer is in traditional Anglican worship, even though that importance has faded (for better or for worse) with the increased Eucharistic practice we have seen in the last several decades. This particular text is a good candidate to “lead off” the hymnal not only because of its venerable origin, but because it was ushered into English-language hymnody by the Anglican all-star team of Dearmer and Vaughan Williams.