Chapter 1, Sutra 10: "Nidra or dreamless sleep is the vrtti produced by cognition of absence or negation." Swami Veda Bharati
Explanation That we can remember that we were sleeping and retain a sense of whether it was a good sleep or not, the state of deep dreamless (NREM) sleep, nidra is a cognition. This means that there is mental awareness during deep sleep, even though it is an awareness of negation. Nidra might be said to be consciousness of being unconscious. This is different from the practice of yoga nidra in which the practitioner remains completely conscious even though in a deep restful state in which the mind produces the same delta waves that it does in nidra.
Because sleep is a cognition, those yogis who wish to experience the true-Self go beyond sleep to attain their goal.