Let us discuss about the Consciousness and how Aum is related to that..
There are 4 types of consciousness as per psychology.
1. Jagrat (Waking consciousness)
2. Svapna (Dreaming Consciousness)
3. Svapna rahit Nidra (Dreamless Sleep Consciousness)
4. Turya or Turiya (Transcendental consciousness)
Now the association of letters A,U and M with these four consciousness is so surprising.... so have patience and read every word with utmost care, so as not to miss the essence.
A - letter represents the Jagrat avastha. That is the consciousness which is present in three dimensional world, hence conditioned by time and space. Our conscious mind is predominating in this state. We mainly feel the mind is swaying of thoughts, impressions and desires. There are three qualities of consciousness, Tamas, Rajas and Sattva and out of these three, Rajas (which is heat and energy) is prevailing in this state.
U - letter represents Svapna avastha. Here, subconscious mind is dominating. We see dreams at this level and those dreams can either be stimulated by nerve cells firing as we lose consicous awareness or they can be the result of deeper level of the psyche and may hold deep symbolic meaning. Here we are allured by the universe and hence, the quality of consciousness prevailing here is TAMAS (lethargy, inertia, ignorance and darkness)
M - letter represents Nidra avastha. The Unconscious mind predominates at this stage as this is a deep sleep state without any disturbance of dreams. This is a blissful state since the ego has been left aside and a connection is experienced with the SOUL. SATTVA (purity, serenity and light) is the main quality of consciousness in this state.
Now, after M we experience silence, often that is called TURIYA avastha. that is the transcendental state, i.e. NON-DUALITY is experienced. This is bit too deep to understand since it says that our being is no longer disconnected from the cosmic consciousness. We are same as the super consciousness or the cosmic consciousness. Our consciousness expands at that level and the BIG MIND predominates in this state.
So, AUM has such a deep meaning of taking us from the conscious level of mind to the TURIYA level and making us unite with our source. At this level, we are fully aware of the activity of our mind and still we are in deep relaxation. Therefore we should chant AUM as a daily practice. and then we should relax to feel that deep connection with the entire CREATION...
One more information about AUM...
“The Whole World is the symbol of AUM, The Past, Present, Future and that which is beyond (Paramatma), These three periods of time are also just “AUM”.” (Mandukya Upanishada)
(Upanishada are ancient scriptures, Vedantas... )