The deep-sleep state, the Cognitional, is the letter m, the third element, from miti ('erecting') or from apīti ('immerging'). He, verily, indeed, erects (minoti) this whole world, and he becomes its immerging—he who knows this.

In plain words: The sound M stands for deep sleep, because it suggests both building up and merging back. Whoever understands this builds up the whole world and becomes the place where it all merges back.