Śaṅkaraon Brahma Sūtra 3.2.8

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 prdwas 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 n^dis, sometimes from the pericardium, sometimes from the Self.—For that reason also the Self is the place of deep sleep.