The first six of its 15 sections are texts from the canonical hour of Vespers.
From the sltrib.com
The 1970 reform of the Divine Office renamed this canonical hour the Office of Readings.
From the en.wikipedia.org
Historically, nocturne is a very old term applied to night Offices and, since the Middle Ages, to divisions in the canonical hour of Matins.
From the en.wikipedia.org
The invitatory psalm is retained only if the Office of Readings is celebrated as the first canonical hour of the day, otherwise it is attached to Lauds.