abhāvapratyayālambanā tamovṛttirnidrā

Deep sleep is when the mind is overcome with heaviness and no other activities are present.

Nidra means deep sleep, the dreamless state in which the mind is focussed on emptiness, void-ness.There are no thoughts arising in this state. The other activities of the mind (imagination, misconception, correct understanding, memory) do not come in to play. Nidra is an independent activity.

The sleep of a common man, where there are so many thoughts, dreams and nightmares are not addressed by Patañjali. I am not sure why. If any of you readers have a different interpretation or if you know where the common man’s sleep is addressed in the sutras, leave a comment.

abhāvapratyayālambanā tamovṛttirnidrā

abhāva -absence, non-existence, non-occurrence, negation, voidness, nothingness
pratyaya – the cause, the feeling, content of mind, presented idea, cognition
ālambanā – support, leaning on, dependent on, having as a base or foundation
tamo – heaviness
vṛtti – operations, activities, fluctuations
nidrā – deep sleep