Hence, Prana and Apana do not desert a sleeping person. On account of its controlling all the vital airs, the controlling breath is so called Udana.
प्राणानामायतत्वेन तमुदानं प्रचक्षते। तस्मात् तपो व्यवस्यन्ति मन्दतं ब्रह्मवादिनः॥