Shloka:
dakṣiṇākṣimukhe viśvo manasyantastu taijasaḥ
ākāśe ca hṛdi prājñastridhā dehe vyavasthitaḥ
Mandukya Karika Shloka 2:
In the waking state, the individual consciousness i.e., the Jiva, observes through the right eye. Why the right eye? Because the ancients had a concept that in the waking state, the centre of consciousness shifts to the right eye. That is the concept of ancients.
In the dream state, that ego shifts to internal memories. Whatever is stored in the memory, it starts experiencing.
In a deep sleep state, there is no other. It rests in its own state. These are the three states: waking, dreaming and deep sleep.
These are given by 3 names, but they are not 3 individuals, they are not 3 separate beings. They are all given only one name which is consciousness for which different names are given. The Sakshi, the witness is called pragna, turiya and taijasaḥ. Same consciousness is called by 3 different names.
References:
Mandukya Karika_Eng_T2_(Shloka2-Consciousness in 3 states)_17Nov2018