Dreamless sleep is an immobile state of consciousness in which the sense of existence is not sensed. This is the very kernel of the tenth yoga sutra of Patanjali Yoga, Abhava pratyaya alambana vrttih nidra.

Abhava non-existence, a feeling of non-being, absence of awareness
pratyaya proceeding towards conviction, trust, confidence, reliance, usage, knowledge, understanding, instrument, means, intellect
alambana support, abode, dependence on a prop, mental exercise to bring before one`s thoughts the gross form of the eternal
vrttih function, condition, thought-wave
nidra sleep without dreams

Sleep is the non-deliberate absence of thought-waves or knowledge.

Sleep is a state in which all activities of thought and feeling come to a close. During sleep, the senses of perception repose in the mind - the mind in the consciousness and the consciousness in the being. Sleep constitutes three kinds. If one feels heavy and dreary after sleep, that sleep has been lamasic. Interrupted sleep is rajasic. Sleep that induces elation, brilliance and freshness is sattvic.

One is awake in the states of correct knowledge, perverse knowledge, fanciful knowledge, and knowledge born from memory. Mind and consciousness are attracted by the senses into contact with external objects - therefore, one attains knowledge. In deep sleep, these four types of knowledge are absent - the senses of perception discontinue to function because their king, the mind, is at rest. This is abhava, a state of nihility, a feeling of emptiness.

The sadhaka, having experienced this negative state of void in sleep, tries to channelise it into a positive state of mind while awake. Then he experiences that pure state in which the self is free from the knowledge of things seen, heard, assumed or felt through the senses and the mind. When he has learned to silence all the modulations of mind and consciousness, then he has accomplished kaivalya. He has sublimated the vrttis and has become a master; his cilia is submerged in the soul.

Sleep furnishes one a glimpse of the seer, but only murkily, because the light of discrimination, viveka, is befogged. Simulation of this state of sleep when one is awake and aware is named samadhi, in which the seer witnesses his own form.