Yoga Sūtra Chapter One verse 10
अभावप्रत्ययालम्बना तमोवृत्तिर्निद्र ॥१०॥
abhāva-pratyaya-ālambanā tamaḥ vṛttiḥ nidrā ||10||
Deep sleep is the fluctuation of obscuring,
supporting a non-appearance of psychic activity.
Commentaries and Reflections
Commentary by T Krishnamacharya:
“Nidrā or deep sleep is the state in which the
mind’s link with external stimuli is cut off.
In this state, Tamas is dominant.
Although in deep sleep the mind
has no link with anything external,
this does not exclude all links,
which is why we are often able to recall
whether our sleep was sound or disturbed.”
Commentary by Paul Harvey:
Deep sleep is the obscuring of the fluctuations,
resulting in the non-appearance of mental activity.