Bhashya
सुषुप्तकाले स्वेन आत्मना सता संपन्नः सन् सम्यक्प्र सीदतीति जाग्रत्स्वप्नयोर्विषयेन्द्रियसंयोगजातं कालुष्यं जहा तीति संप्रसादशब्दो यद्यपि सर्वजन्तूनां साधारणः, तथापि एवंवित् स्वर्गं लोकमेतीति प्रकृतत्वात् एष संप्रसाद इति संनिहितवद्विशेषात् सः अथेदं शरीरं हित्वा अस्माच्छ रीरात्समुत्थाय शरीरात्मभावनां परित्यज्येत्यर्थः ।न तु आसनादिव समुत्थायेति इह युक्तम्, स्वेन रूपेणेति विशे-तानि ह वा एतानि ब्रह्मणो नामाक्षराणि त्रीण्येतानि सतीयमिति, सकारस्तकारो यमिति च ।ईकारस्तकारे उच्चारणार्थोऽनुबन्धः, ह्रस्वेनैवाक्षरेण पुनः प्रतिनिर्देशात् ।तेषां तन् तत्र यत् सत् सकारः तदमृतं सद्ब्रह्म—अमृतवाचकत्वादमृत एव सकारस्तकारान्तो निर्दिष्टः ।अथ यत्ति तकारः तन्मर्त्यम् ।अथ यत् यम् अक्षरम्, तेनाक्षरेणामृतमर्त्याख्ये पूर्वे उभे अक्षरे यच्छति नियमयति वशीकरोत्यात्मनेत्यर्थः ।यत् यस्मात् अनेन यमियेतेन उभे यच्छति, तस्मात् यम् ।संयते इव हि एतेन यमा लक्ष्येते ।ब्रह्मनामाक्षरस्यापि इदममृतत्वादिधर्मवत्त्वं महाभाग्यम्, किमुत नामवतः—इत्युपास्यत्वाय स्तूयते ब्रह्म नामनिर्वचनेन ।एवं नामवतो वेत्ता एवंवित् ।अहरहर्वा एवंवित्स्वर्गं लोकमेतीत्युक्तार्थम् ॥
In plain words
In deep sleep, united with his true Self as pure Being, a person finds profound peace, leaving behind the restlessness caused by waking and dreaming senses. Although the term 'serene one' generally applies to all sleeping creatures, here it specifically designates the enlightened knower, as established by the context. Leaving this body means abandoning the false identification with the physical form—not standing up physically as from a chair. Brahman's name consists of three syllables: sat, ti, and (y)am, where the vowel i on ta is merely an auxiliary aid for pronunciation. Of these, 'sat' (represented by sa) is immortal Existence-Brahman. 'Ti' (represented by ta) is the mortal aspect. The syllable 'yam' is that through which the Self governs and holds both the immortal and mortal aspects within itself; because it restrains them, it is called 'yam'. If even the syllables forming Brahman's name possess such glorious attributes of immortality, how much more glorious is the supreme Lord who bears that name! Thus, scripture praises Brahman through its etymological derivation to inspire meditation. The meaning of the statement that the knower thus attains the luminous realm day by day has already been explained.
Close to the Sanskrit
During deep sleep, being united with his own Self as Being, he becomes thoroughly peaceful—because he abandons the turbidity produced by the contact of the senses with objects in waking and dreaming, although the term 'serene one' is common to all creatures, nevertheless, because it was stated 'the knower thus attains the heavenly world', this 'serene one' [is understood here] due to contextual proximity and specific distinction. Now, having left this body, having risen from this body—leaving behind the notion that the body is the self, not having risen up like from a seat, as is appropriate here... These three syllables of Brahman's name are indeed sat, ti, and yam. The vowel ī attached to ta is an auxiliary adjunct for pronunciation, because of subsequent re-reference with a short vowel only. Among them, whatever is 'sat', that is the letter sa; that is immortal, Existence-Brahman—since it denotes immortality, 'sa' is indicated ending with 'ta'. Now, whatever is 'ti', that is the letter ta, which is mortal. Now, whatever is the syllable 'yam', by that syllable he restrains, regulates, and brings under control within himself both prior syllables named immortal and mortal. Because by this 'yam' he controls both, therefore it is named 'yam'. As if controlled, indeed, both appear by this. The possession of such attributes as immortality by the syllable of Brahman's name is a great glory; how much more so of the possessor of the name—thus Brahman is glorified through the etymological definition of its name. Thus the knower of the possessor of the name is an 'evam-vid' [knower thus]. 'Day by day indeed the knower thus attains the heavenly world' has its meaning already explained.