Art by smbc-comics
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
Latest studies with FMRIs and other tools have found that general anesthesia decouples the sections of the brain from each other. All the various parts of the brain stop communicating. It's an entities different state than sleep based on the brain activity.
Normally when we have various stimuli or are asleep, neural activity "flows" around from one section to the other. When under general anesthesia those flows are isolated and don't connect to other sections of the brain.
This has actually given us a huge clue as to where consciousness comes from and what makes it a thing.
It also helps explain why going under is just lights out and no drama or anything. It's like an off switch for the "person".