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Consciousness is often said to disappear in deep, dreamless sleep. We argue that this assumption is oversimplified. Unless dreamless sleep is defined as unconscious from the outset there are good empirical and theoretical reasons for saying that a range of different types of sleep experience, some of which are distinct from dreaming, can occur in all stages of sleep.
Pubmed Articles
Does Consciousness Disappear in Dreamless Sleep?
Sciencealert Article We Were Wrong About Consciousness Disappearing in Dreamless Sleep, Say Scientists
It's way more than just that, though. You're also disconnected from your sensory inputs, and furthermore, your conscious experience is interrupted. It's not like you're just in a sensory deprivation tank, because there you'd still experience conscious thought, and the passage of time. It just seems to turn off for a while.
Plus there are periods of deep sleep when your brain does shut down quite thoroughly. People just don't remember those, obviously, so they put a lot more weight on the dreaming bits that slip through sometimes.