Yoga sutra 1.10
Abhāva-pratyayālambanā tamovrttih nidrā
Deep sleep is a mental state based on the absence of other mental activity
Deep sleep (nidrā) occurs in the absence of other mental activities such as correct knowledge (pramāna), misconception (viparyaya) or imagination (vikalpa). It is different to REM sleep which is part of imagination. Only when you are fast asleep, deep asleep, the contents of the mind stop. The mind disappears, and you are in yourself without any content.
Damian Cadman-Jones is an authorised teacher of Prana Vashya Yoga™