The Yoga Sūtra clarifies this very well.

abhāva-pratyaya ālambanā nidrā vṛtti (1.10)

This clarifies that deep sleep (nidrā) is a mental function (vṛtti), which makes one aware of non-being (abhāva-pratyaya) – in other words it is the experience of not experiencing anything except the basic underlying capacity to experience (ālambāna).

Because it is a mental function it is not the goal of yoga, i.e. it is not direct perception of Brahman. Therefore it is indeed a product of material circumstances (prākṛtika-guṇa), notably the circumstance of “darkness” (tamo-guṇa). However, because it is an experience of the underlying capacity to experience, yet is devoid of any specific experiential detail it is therefore a state that is in some significant way similar to the experience of Brahman.