The Four Parts of Om
A-U-M and the silence correspond to the four states.
The syllable Om has four parts: A corresponds to waking, U to dreaming, M to deep sleep, and the silence that follows to Turiya - the transcendent fourth.
A is the waking state, U is the dream state, M is the deep sleep - the fourth is the silence.
This analysis reveals the complete teaching of the Self.
Mandukya Upanishad