Mandukya Upanishad: From: Atharva Veda

Mandukya Upanishad is a major Upanishad from Atharva Veda. This Upanishad “contains the epitome of the substance of the import of Vedanta.” It sets forth the Vedantin theory of consciousness: waking, dreaming, deep sleep, and the fourth state (turiya), the Self.