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.

abhāva - non-appearance, non-existence, absencepratyaya - psychic activity; cause; conception, assumption, notion, idea;ālambana - supporting, resting ontamas - obscuring; mental darkness, ignorance, illusion, error; deprived of the eye's light or sight; darkness, gloomvṛtti - fluctuation; modification; turning; set in motion, course of action, behaviour, movement; activity, function; professionnidrā - deep sleep; slumber; sleepiness; sloth

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.

Inspirational Quote

“Happiness, it seems to me, consists of two things: first, in being where you belong, and second -- and best -- in comfortably going through everyday life, that is, having had a good night's sleep and not being hurt by new shoes....” Theodor Fontane