The Night Office of the Monastic Breviary according to the Holy Rule of St. Benedict.

The word "matins" is derived from Latin adjective matutinus, meaning "of or belonging to the morning".[26] It was at first applied to the psalms recited at dawn, but later became attached to the prayer originally offered, according to the fourth-century Apostolic Constitutions, at cockcrow and, according to the sixth-century Rule of Saint Benedict, at could be calculated to be the eighth hour of the night (the hour that began at about 2 a.m.).

Between the vigil office and the dawn office in the long winter nights there was an interval, which "should be spent in study by those [monks] who need a better knowledge of the Psalter or the lessons"; in the summer nights the interval was short, only enough for the monks to "go out for the necessities of nature". The vigil office was also shortened in the summer months by replacing readings with a passage of scripture recited by heart, but keeping the same number of psalms. Both in summer and in winter the vigil office was longer than on other days, with more reading and the recitation of canticles in addition to the psalms.

Outside monasteries few rose at night to pray. The canonical hour of the vigil was said in the morning, followed immediately by lauds, and the name of "matins" became attached to the lengthier part of what was recited at that time of the day, while the name of "lauds", a name originally describing only the three Psalms 148-150 recited every day at the end of the dawn office (until excised in the 1911 reform of the Roman Breviary by Pope Pius X; see Lauds), was applied to the whole of that office, substituting for the lost name of "matins" or variants such as laudes matutinae (morning praises) and matutini hymni (morning hymns). An early instance of the application of the named "matins" to the vigil office is that of the Council of Tours in 567, which spoke of ad matutinum sex antiphonae.

The Rule of Saint Benedict clearly distinguished matins as the nighttime hour, to which he applied Psalm 118/119:62, "At midnight I rise to praise you, because of your righteous rules".

For monastics, the canonical hour begins with the versicles, "O God, come to my assistance; O Lord, make haste to help me" and "O Lord, Thou wilt open my lips, and my mouth shall declare Thy praise" (the latter said three times) followed by Psalm 3 and Psalm 94 (the Invitatory). The Invitatory is to be recited slowly out of consideration for any late-arriving monk, since anyone appearing after its conclusion is traditionally punished by having to stand in a place apart. After this a hymn is sung.

Next comes two sets of six psalms followed by readings. (What we know call nocturns.) The first set is of six psalms followed by three readings from the Old or New Testaments or from Church Fathers (N.B. the Readings are not included in this publication). Each reading is followed by a responsory. The second set of six psalms is followed by a passage from the Apostle Paul recited by heart and by some prayers. The Night Office then concludes with a versicle and a litany that begins with Kyrie eleison.

Since summer nights are shorter, from Easter to October a single passage from the Old Testament, recited by heart, takes the place of the three readings used during the rest of the year.

On Sundays, the office is longer, and therefore begins a little earlier. Each set of six psalms is followed by four readings instead of three after the first set and a single recitation by heart after the second set. Then three canticles taken from Old Testament books other than the Psalms is recited, followed by four readings from the New Testament, the singing of the Te Deum, and a reading by the abbot from the Gospels (Sunday Gospel readings are included in this publication).