As I discuss in this question, by far the most popular school of Hindu philosophy is the Vedanta school, which bases its tenets on the doctrines laid out in the Brahma Sutras, a work by the sage Vyasa that summarizes and systematizes the philosophical teachings of the Upanishads. You can read the Brahma Sutras here. In any case, in the beginning of Adhyaya 3 Pada 2 of the Brahma Sutras, Vyasa discusses the nature of different states of consciousness. Here is what he says about Sushupti or deep sleep:
Topic-2: The Soul in Deep Sleep
- The absence of that dream (ie., dreamless sleep) takes place in the nerves and the Self, as it is known to be so from the Upanishads.
- For the same reason, the soul’s waking up is from this supreme Self.
As I discuss in this answer, all the commentators on the Brahma Sutras interpret these Sutras as saying that during deep sleep, the Jiva resides in Brahman. Here is what Adi Shankaracharya says in his Brahma Sutra Bhashya, for instance:
'The absence of that,' i.e. the absence of dreams--which absence constitutes the essence of deep sleep-takes place 'in the nâdîs and in the Self;' i.e. in deep sleep the soul goes into both together, not optionally into either.--How is this known?--'From scripture.'--Scripture says of all those things, the nâdîs, &c., that they are the place of deep sleep; and those statements we must combine into one, as the hypothesis of option would involve partial refutation.... That again the nâdîs and the pericardium have to be combined as places of deep sleep appears from their being mentioned together in one sentence ('Through them he moves forth and rests in the purîtat). That that which is (sat) and the intelligent Self (prâgña) are only names of Brahman is well known; hence scripture mentions only three places of deep sleep, viz. the nâdîs, the pericardium, and Brahman. Among these three again Brahman alone is the lasting place of deep sleep; the nâdîs and the pericardium are mere roads leading to it. Moreover (to explain further the difference of the manner in which the soul, in deep sleep, enters into the nâdîs, the pericardium and Brahman respectively), the nâdîs and the pericardium are (in deep sleep) merely the abode of the limiting adjuncts of the soul; in them the soul's organs abide.
Nor do we finally maintain that the nâdîs, the pericardium, and Brahman are to be added to each other as being equally places of deep sleep. For by the knowledge that the nâdîs and the pericardium are places of sleep, nothing is gained, as scripture teaches neither that some special fruit is connected with that knowledge nor that it is the subordinate member of some work, &c., connected with certain results. We, on the other hand, do want to prove that that Brahman is the lasting abode of the soul in the state of deep sleep; that is a knowledge which has its own uses, viz. the ascertainment of Brahman being the Self of the soul, and the ascertainment of the soul being essentially non-connected with the worlds that appear in the waking and in the dreaming state. Hence the Self alone is the place of deep sleep.
And here is what Ramanujacharya says in his Sri Bhashya:
Next the state of deep dreamless sleep is enquired into. Scripture says, 'When a man is asleep, reposing and at perfect rest, so that he sees no dream, then he lies asleep in those nâdîs' (Kh. Up. VIII, 6, 3); 'When he is in profound sleep and is conscious of nothing, there are seventy-two thousand veins called hita which from the heart spread through the pericardium. Through them he moves forth and rests in the pericardium' (Bri. Up. II, 1, 19). 'When a man sleeps here, he becomes united with the True' (Kh. Up. VI, 8, 1). These texts declare the veins, the pericardium, and Brahman to be the place of deep sleep; and hence there is a doubt whether each of them in turns, or all of them together, are that place. There is an option between them, since they are not in mutual dependence, and since the sleeping soul cannot at the same time be in several places!--To this the Sûtra replies--the absence of dreams, i.e. deep sleep takes place in the veins, in the pericardium, and in the highest Self together; since these three are declared by Scripture. When different alternatives may be combined, on the ground of there being different effects in each case, it is improper to assume an option which implies sublation of some of the alternatives. And in the present case such combination is possible, the veins and the pericardium holding the position of a mansion, as it were, and a couch within the mansion, while Brahman is the pillow, as it were. Thus Brahman alone is the immediate resting-place of the sleeping soul.
See my answer here for quotes from other commentries saying the same thing. But my question is, what is the purpose of the Jiva residing in Brahman during deep sleep?
Note that I am not asking how we know that the Jiva resides in Brahman during deep sleep; that is thoroughly addressed in the various commentaries on the Brahma Sutras. In some sense I'm interested in the purpose of the deep sleep state. Not the scientific or health purpose of sleep, but rather, what is the benefit of the Jiva going into a state every night where it resides in Brahman? What purpose does that serve?
Are there any scriptures or works by Acharyas that address this?