> But from the inside observed by oneself, it feels quite similar

Having been under general anesthesia once the feeling of just completely not existing for the time period I was under felt very different than sleep does to me.

Perhaps it's because I almost never dream. Sleep feels almost just as dead to me.

Case in point, I think we're probably talking about slightly different phenomena even when we talk about the same linguistic definition of consciousness. Kind of like how my idea of an "apple" is based on all of the apples I've seen in the past and instances where I've used the word or saw it being used by others, whereas your idea of "apple" is based on your experiences. They must be mostly similar in information content or we wouldn't be able to communicate at all, but the self-or-other distinction is important.