The works of Ādi Śaṅkarācārya

Bṛhadāraṇyakopaniṣadbhāṣyam

Verse 214

तदेते श्लोका भवन्ति ।स्वप्नेन शारी- रमभिप्रहत्यासुंसः सुसानभिचाकशीति ।शुक्रमादाय पुनरैति स्थानँ हिरण्मयः पुरुष एकहंसः ॥ ११ ॥

In plain words

And so there are these verses about it: 'Through dream he lays the body down, and, never sleeping himself, watches over all that sleeps. Then, taking up that pure bright essence, he returns once more to his place -- the golden person, the one swan.'

Close to the Sanskrit

Then there are these verses: 'Having struck down the bodily frame by means of dream, the unsleeping one looks upon the sleeping [faculties]. Having taken the bright [essence], he goes again to his place — the golden person, the one swan.'