What is meant by "the depths of the last part of the night" and when does it begin by the hour?

Ibn Manzur says in Lisan al-Arab: In the Hadith, he (the Prophet, peace be upon him) was asked: "Which part of the night is most conducive to hearing (prayer)?" He replied: "The last part of the middle of the night," meaning its last third, which is the fifth part of the sixths of the night.