avyaktād vyaktayaḥ sarvāḥ
prabhavanty ahar-āgame
rātry-āgame pralīyante

Translation of Bhagavad Gita 8.18

At the beginning of Brahma’s day, all living entities become manifest from the unmanifest state, and thereafter, when the night falls, they are merged into the unmanifest again.

Commentary by Sri A.C. Bhaktivedanta Swami Prabhupada of Gaudiya Sampradaya:

No commentary by Srila Prabhupada.

Commentary by Sri Vishvanatha Chakravarthi Thakur of Gaudiya Sampradaya:

Those who are inferior,  residing within the three planetary systems, perish day after day. Madhusudana Sarasvati says: the word avyakta refers to Brahma in his sleeping state (not a total dissolution), since during the daily creation and dissolution of the universe, the elements remain intake. Then from a sleeping state (avyaktat), through Brahma, the places of enjoyment in the form of bodies and objects of enjoyment manifest (vyaktaya), or become suitable for use. At the coming of night, when Brahma sleeps, everything disappears within him.

Commentary by Sri Ramanuja of Sri Sampradaya:

8.18 Thus, at the dawn of a day of Brahma, the manifest entities existing in the three worlds, possessing body, senses, objects, and places of enjoyment appear from the non-manifest (Avyakta), which is the condition of Brahma’s body in that state, and at the beginning of the night they are dissolved into the condition of the unevolved (Avyakta) which forms the body of Brahma then.

Commentary by Sri Sridhara Swami of Rudra Sampradaya:

What subsequently is the process? Lord Krishna is answering. From the unmanifest form of the effect is the cause. Therefore from the unmanifest which is the cause, all created beings, moving or stationary come into existence. When is this? At the commencement of Brahma’s day every 4 billion 320 million years and at the end of Brahma’s day all created beings are merged back into the unmanifest for 4 billion 320 million years. This verse clarifies that the knowers of day and night also know the cycle of the unmanifest and the manifest and what happens to all created beings.

Commentary by Sri Madhvacharya of Brahma Sampradaya:

Sri Madhvacharya did not comment on this sloka.

Commentary by Sri Keshava Kashmiri of Kumara Sampradaya:

Thus Lord Krishna previously described that created beings from the highest material planet Maharloka of Brahma down to the lower heavenly planets called Mahat are all subjected to rebirth in the material existence. Now He explains that the creation and dissolution of all the material worlds like Earth, the galaxy, our solar system, all the universes up through the heavenly planets to the topmost planet of Maharloka are all created and absorbed back into the avyakta or the unmanifest which is regulated by Brahma’s day and night each comprised of 4 billion 320 million years.

All sentient beings in physical existence were created by Brahma from his subtle body. This subtle body is one of the transformations of prakriti or the material substratum pervading physical existence. This the unmanifest is from Brahma and is within material existence although super subtle and from which emanates countless myriad of unlimited beings accompanied by their respective bodies and minds and senses with which to enjoy their respective activities and pleasures. This happens at the dawning of Brahmas day when he awakes. At the closing of Brahma’s day and the beginning of his night all beings who emanated from his subtle body are withdrawn again into his subtle body to repose in dormancy until his awakening again after 4 billion 320 million years.

Commentary by Sri Adi Shankaracharya of Advaita Sampradaya:

8.18 Ahar-agame, with the coming of day, at the time when Brahma wakes; sarvah vyaktayah, all manifested things, all things that get manifested, all creatures characterized as moving and non-moving; prabhavanti, emerge, become manifested; avyaktat, from the Unmanifested-avyakta (Unmanifested) is the state of sleep of Prajapati; from that avyakta. Similarly, ratri-agame, when night comes, at the time when Brahma sleeps; praliyante, they, all the manifested things, merge; tatra eva, in that itself; avyakta-sanjnake, which is called the Unmanifested referred to above. In order to obviate the defect of the emergence of some unmerited result and the destruction of merited results; [The following verse says that the very same multitude of beings continues in the different cycles of creation, and there-fore these two defects do not arise.] for pointing out the meaningfulness of the scriptures [For the earlier reason the scriptures do not lose their validity.] dealing with bondage and Liberation; and with a view to propounding detachment from the world on the ground that the helpless multitude of beings perishes after being born again and again under the influence of accumulated results of actions that have for their origin such evils as ignorance etc. [The five evils are: ignorance, egoism, attachment, aversion and clinging to life. (See P. Y. Su. 2.3)], the Lord says this:

Commentary by Sri Abhinavagupta of Kaula Tantra Sampradaya:

8.17-19 Sahasra-etc., upto aharagama. Those who could see afar (great seers), see [actually] the night and day even in the case of Brahma as being marked [respectively] by the destruction and creation [of the world]. Accordingly, having risen from sleep, the same [Selves] continue their own respective activities every day; they theyselves, putting an end to their activities every night, remain exclusively in the form of Energy [of the Absolute]. In this manner they come to be again and again at the time of creation and of dissolution. No new, but only the self-same personal Souls are let loose. Their mutual difference in the form of the idea of the long and short lives is based only on the concept of time. This delimitation is unavoidable even in the case of the Prajapatis. Hence it is established that they too are cetainly of the nature of having evolution and dissolution. [The Lord] clarifies His [own] statement : ‘People do return from each and every world; but having attained Me, the Supreme Lord, they do not do so.’

Sanskrit Shloka Without Transliteration Marks:

avyaktad vyaktayah sarvah
prabhavanty ahar-agame
ratry-agame praliyante

Sanskrit to English Word for Word Meanings:

avyaktāt — from the unmanifest; vyaktayaḥ — living entities; sarvāḥ — all; prabhavanti — become manifest; ahaḥ-āgame — at the beginning of the day; rātri-āgame — at the fall of night; pralīyante — are annihilated; tatra — into that; eva — certainly; avyakta — the unmanifest; saḿjñake — which is called.