core concepts and philosophy
What is the fourth state of consciousness (turiya) and how does it differ from deep sleep?
The four states
Upanishadic thought describes four states the self moves through. Waking is the everyday state, where we experience the outer world. Dreaming is the inner world of images and stories during sleep. Deep sleep, called sushupti, is a state of rest with no dreams and no sense of a separate self. Then there is turiya, which means simply 'the fourth.' It is not a state you fall into the way you fall asleep. The tradition describes it as the ground that the other three states rest on. It is always present, underneath the other three.
How turiya differs from deep sleep
This is the key difference. In deep sleep, awareness goes blank. There is rest, but no knowing. When you wake from deep sleep, you remember nothing. Turiya is the opposite of blank. It is described as pure witnessing, a wide-open awareness with no content, no dreamer, and no sleeper. Deep sleep is like a covered lamp. Turiya is the light itself. The tradition says most people pass through something like turiya every night on the way into and out of deep sleep, but do not notice it because the mind is too dull or too busy. Recognising it clearly is what the tradition calls awakening.
Where this teaching comes from
This framework comes from the Mandukya Upanishad, one of the shorter Upanishads and one of the most concentrated in its teaching. It maps the syllable Om onto the four states. Later, a thinker named Gaudapada wrote a commentary that developed these ideas further, arguing that turiya is not really a fourth state added to the other three but the nature of the self all along. His work became very important in the Advaita, or non-dual, stream of Hindu thought.
How people engage with it today
For many people this teaching is philosophical, a way of understanding what the self really is. For others it connects to meditation practice, where the aim is to stay aware even as the mind grows quiet. Some teachers describe deep meditation as touching turiya while awake. Scientists study sleep states and have mapped the brain during waking, dreaming, and dreamless sleep, but turiya as the tradition describes it, a witnessing awareness with no object, does not map neatly onto anything measured so far. The tradition itself says it cannot be fully captured in words, only pointed at.