“Sleep and Samādhi are the only times when there is no ‘I‘.”
– TKV Desikachar on Yoga Sūtra Chapter One verse 2

“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.”
– T Krishnamacharya on Yoga Sūtra Chapter One verse 10

“The state of Dhyānam is possible in a seated posture.
If a person lies down, it may induce sleep.
If a person walks and moves about,
he may be distracted by the objects around him.
This posture must be in a place
where the mind will not be distracted.”
– T Krishnamacharya on Yoga Sūtra Chapter Three verse 2