Vedānta examines the three states of experience to show that the self is none of them. In waking the self knows outer objects, in dreaming inner ones, and in dreamless sleep nothing in particular, yet on waking one recalls having slept well. What persists through all three is the witness consciousness, called the fourth, turīya.
Avasthā-traya
The three states of waking, dreaming and dreamless sleep, analysed in Vedānta to discover the self.
Also: three states