The distinction between a man absorbed in contemplation on
Brahman and the man who is in deep sleep should be clearly understood; in
contemplation the mind is kept under control and does not lie defunct as it
were like the mind of a man in deep sleep This is really the fearless state of
Brahman with the rays of knowledge shining all round.
It has been stated that with the attainment of knowledge
that Atma alone is real, the mind becomes devoid of all hopes and doubts, and
like fire without firewood it becomes extinct and remains under control.
It is evident that when mind is unminded there will be no
duality. Yogis should understand the nature of mind, when it is kept under
control and is made defunct with true knowledge. It might appear that the mind
of a Yogi is like that of a man in deep sleep both being almost defunct and
there being apparently no distinction between them.
But it is not really so; the mind of a Yogi differs from
that of a man in sound sleep in that the latter is saturated with ignorance,
the cause of various vasanas and harmful notions burnt in the fire of Brahman
knowledge and is free from all kinds of Kleshas. Hence they are not similar. The
reason why they differ is that while the mind of a man in sleep is saturated
with the notions of Avidya and lies down with ignorance in utter darkness, the
mind of a Yogi kept under control with true knowledge does not become the seed
of ignorance. When it is free from all blemish due to the notion of the knower
and knowable, it becomes fearless Brahman itself, for in that state there is no
notion of duality, the cause of fear. It is duality that is the cause of all
fear.
Source - Adi Shankaracharya Commentary On Upanishads