The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
8. Hence the awaking from that (viz. Brahman).
And because the Self only is the place of deep sleep, on that account the scriptural chapters treating of sleep invariably teach that the awaking takes place from that Self. In the Bri. Up. when the time comes for the answer to the question, 'Whence did he come back?' (II, 1, 16) the text
says, 'As small sparks come forth from fire, thus all prânas come forth from that Self (II, 1, 20). And Kh. Up. VI, 10, 2, we read: 'When they have come back from the True they do not know that they have come back from the True.' If there were optional places to which the soul might resort in deep sleep, scripture would teach us that it awakes sometimes from the nâdîs, sometimes from the pericardium, sometimes from the Self.--For that reason also the Self is the place of deep sleep.