Arstawakr means that you are the witness without any practice. The state of deep sleep is important for a twitveranta. It's an important thing for adwetaveante because doubts usually come there. That which is trying to practise is the mind. And it fails most spectacularly in deep sleep, because the mind goes to sleep.
Inspired by the 15th chapter of the Ashtavakra Gita, Swami Sarvapriyananda gives a discourse on the nature of the Self.