Praśna Upaniṣad · 4..2
तस्मै स होवाच यथा गार्ग्य मरीचयोऽर्कस्यास्तं गच्छतः सर्वा एतस्मिंस्तेजोमण्डल एकीभवन्ति ताः पुनः पुनरुदयतः प्रचरन्त्येवं ह वै तत्सर्वं परे देवे मनस्येकीभवति तेन तर्ह्येष पुरुषो न शृणोति न पश्यति न जिघ्रति न रसयते न स्पृशते नाभिवदते नादत्ते नानन्दयते न विसृजते नेयायते स्वपितीत्याचक्षते
tasmai sa hovāca yathā gārgya marīcayo'rkasyāstaṃ gacchataḥ sarvā etasmiṃstejomaṇḍala ekībhavanti tāḥ punaḥ punarudayataḥ pracarantyevaṃ ha vai tatsarvaṃ pare deve manasyekībhavati tena tarhyeṣa puruṣo na śṛṇoti na paśyati na jighrati na rasayate na spṛśate nābhivadate nādatte nānandayate na visṛjate neyāyate svapitītyācakṣate
To him he replied: As the rays of the sun, Gārgya, when it sets, all unite in that disk of light, and they go forth again when it rises; thus, verily, all that unifies in the superior deity, the mind. Therefore then this person does not hear, does not see, does not smell, does not taste, does not touch, does not speak, does not grasp, does not enjoy, does not emit, does not move. “Sleeps”, thus they say.
The metaphor of the sun’s rays (marīcaḥ) withdrawing at sunset (astaṃ gacchati) illustrates the sleep process. When the sun sets, its rays withdraw into the solar disk; when the sun rises, the rays extend again.
Thus, in deep sleep (suṣupta), all the senses unify (ekībhavanti) in manas — the mind, the “superior deity” (paraḥ devaḥ). The ten senses (five of knowledge, five of action) cease to function.
The enumeration of the ten functions that cease is exhaustive. This is svapna — the dream state, when the mind has withdrawn from external objects and operates only with its own contents.
For the yogī, this shows that the mind is the origin of all experience. In deep meditation (samādhi), as in sleep, the senses withdraw (pratyāhāra), allowing introspection.