Sri Aurobindo translates Prashna Upanishad, Question 4, Verses 3-4: “But the fires of the breath keep watch in that sleeping city. The lower breath is the householder’s fire and the breath pervasor the fire of the Lares that burneth to the southward. The main breath is the orient fire of the sacrifice: and even as the eastern fire taketh its fuel from the western, so in the slumber of a man the main breath taketh from the lower. But the medial breath is the priest, the sacrificant: for he equaliseth the offering of the inbreaeth and the offering of the outbreath. The Mind is the giver of the sacrifice and the upper breath is the fruit of the sacrifice, for it taketh the sacrificer day by day into the presence of the Eternal.”
There is a symbolic meaning to each of the types of sacrificial fire used in the vedic sacrifices. Here the Rishi is drawing a correspondence between each of the 5 primary “breaths” and the various sacrificial fires, showing the parallel once again between the inner and the outer. It is not easily possible to determine the exact intention of the Rishi here beyond symbolic correspondences. The same prana responsible for carrying the soul from birth to birth, udana, is here said to carry the mind to the Eternal during deep sleep.
While the individual sleeps, life goes on. The pranas remain active, or “keep watch” as the Rishi declares. The active external functioning may be reduced, but the basic internal functions of respiration, circulation, internal organ functionality and some observation of certain senses continue and allow the body to undertake repairs and maintenance, provide rest to the body and the mind, while at the same time allowing the active conscious intelligence to disassociate itself from the external world temporarily.
Sri Aurobindo, The Upanishads, Prashna Upanishad, pp.297-315