The self is known in four states,the waking,the dreaming,the deep-sleep state and the fourth state
called the Turiya (Absolute). All these states are represented in the three sounds A U M and
the silence that follows and sorrounds the syllable.

A represents the waking state,
U represents the dream state,
M represents deep-sleep state.

The waking state is the first of the three states of consciousness The dream state occurs between the waking and the deep- sleep state and comes second among the three.‘M’ is the closing sound of the syllable and thus the comparison between the deep-sleep state or the final stage of the mind.Between the two Oms a silence is inevitable and that is the fourth state ’Turiya’ of perfect bliss when the individual recognises its identity with the supreme.

Atharva Veda relates a story of Indra where he overcame the demons with the help of Om.
Man can conquer his beastly nature withthe repetition of Om.

Yajurveda exhorts us to try to realise Brahman through
repeating and remembering Om.

Kathoupanishad declares Parabrahman itself is Om

Mundakopanishad advises the spiritual aspirant to meditate on
the unity of the Atman (self) with Brahman (God) using Om.

Sri Krishna avers that He is Om .

Patanjali Yoga Sutras declare that Pranava is the symbol of God and that one can
attain samadhi by its repitition and meditation on Him.All religious rites are started
with the repitition of Om. At death if one succeeds in chanting Om
simultaneously thinking of God he will attain the highest truth.