if you constantly bring your samyama to breathing,
by and by you will come to a state
where you will see breathing has almost stopped.
Yogis do this meditation facing a mirror
because the breathing becomes so silent,
they cannot feel it. They can only see
whether there is some mist on the mirror
from their breathing or not.
Sometimes they have become so silent
that they cannot decide
whether they are still alive or not.
In deep meditation this experience will happen
to you also sometimes. Don’t be afraid.
Breathing almost stops.
When consciousness is perfect,
breathing almost stops, but don’t be worried.
That is not death; that is simply stillness.
Osho, Yoga: The Alpha and the Omega Vol 8, Ch 3 (excerpt)