ओंकारोच्चारणप्रान्तशब्दतत्त्वानुभावनात् ।सुषुप्ते संविदो जाते प्राणस्पन्दो निरुद्ध्यते
In plain words
From directly realizing the very essence of sound at the furthest edge of the utterance of Oṃ; when saṃvid has come into the state of deep sleep -- the vibration of prāṇa is restrained.