"Matins is the monastic nighttime liturgy, ending at dawn, of the canonical hours. In the Roman Catholic pre-Vatican-II breviary, it is divided into three nocturns. The name "matins" originally referred to the morning office also known as lauds. When the nocturnal monastic services called vigils or nocturns were joined with lauds, the name of "matins" was applied at first to the concluding morning service and later still to the entire series of vigils." http://en.wikipedia.org/wiki/Matins
Musically speaking, matins (mornings) refers to a composition of sacred character intended to be performed from late night until before dawn. Its structure is quite simple: An introduction (Deus in adjutorium), ensuing by an alternation of antiphonies sung in plain chant and responsories constructed in a more elaborated way sung with instrumental accompaniment by soloists. For finishing is sung the salm "Te DEUM". In this recording were added intentionally two works of Sumaya; as interludio "Albricias mortales" (mortals, good news) and as ending "Angélicas milicias" (heavenly militias).