The teaching on the four states of consciousness or four levels of being is one of the main tenets of Advaita philosophy.
It is set out clearly in the Mandukya Upanishad which describes three levels of experience, waking, dreaming and deep sleep, through which the individual soul passes when moving from its ordinary, day time state to union with the fourth state, Brahman. This Upanishad makes a connection between sound and levels of consciousness, relating the sounds a, u and m to three realms of experience, waking, dreaming and deep sleep respectively.
In the Indian tradition sound is understood as the creative energy through which the whole Universe comes into being. Aum, or Om, is considered to be the most prevalent sacred sound, syllable and symbol for all that exists whether past, present or future. Everything in the created world originates in Om which is essentially vibrating energy having a subtle form composed of sound. Om is the mystery of the divine.
The way we experience the four states of being may be described as follows: the waking state is the experience of reality common to all which we may call the daydream state. Attention faces outwards. When we let go of the daydream or waking state the world comes to an end. Attention faces inwards and we enter the nightdream state. During our sleep, in our dreams, we are preoccupied with subtler realms and our experiences in the waking state are replaced by symbolic expression. A lot of activity may take place in the dream state. When the waking state is transcended it is seen to be a dream and the dream state, a dream within a dream. Both states are projections of our minds onto the inner screen of our soul.
The daydream of the waking state appears to last longer than the dream state. Thus we may deduce another aspect of the levels of being. Not only do they relate to sound and to vibration but they also relate to different durations of time, to different rates of vibration which become increasingly fine.
We also have a third state, the dreamless state of deep sleep. This state is of two types: one is when we sleep without any recollection of activity and the other is when we enter deep meditation. In the latter state we are disconnected from the world but fully aware of it. There is pure awareness but in deep sleep there is ignorance.
The fourth state transcends all three levels of time. The fourth level is that of the pure Self, of Truth, of Om. This is the mysterious art of the Divine.