1701- AUM

OM represents the Self which is the Supreme Non-dual Reality. 
The Self is known in four states, namely, the waking state, the dream state, the deep-sleep state, and the fourth state called the turiya. All these states are represented in the three sounds of OM (i.e., A, U, M), and the silence that follows and surrounds the syllable.
 
The sound A represents the waking state; 
the sound U represents the dream state, and 
the sound M represents the deep sleep state. 
The waking state is superimposed on the A sound because it is the first of the three states of consciousness, and so is the sound A the very first of the letters of the alphabet – in all languages. The dream is but a view within the mind of the impressions that had reflected on the surface of the mental lake during the waking state.
 Besides, the dream state occurs between the waking and the deep-sleep state, and comes second among the three states of consciousness. And so, U, being next to A in order of sounds, and also since it is between A and M, is treated as representing the dream state. 
 On the M sound of OM is superimposed the deep-sleep state. The comparison between the last sound of the OM and sleep lies in that it is the closing sound of the syllable, just as deep sleep is the final stage of the mind in rest. 
A short pregnant silence is inevitable between two successive OMs. On this silence is superimposed the idea of the fourth state, known as turiya. This is the state of Perfect Bliss when the individual Self recognises the identity with the Supreme.