I doubt anyone is able to fully answer this as everyone who has done it so far hasn't survived to tell the story.

I imagine that it would vary from case to case depending on the actual cause of death, but I believe that passing peacefully while sleeping is definitely something that can happen

Depends on where you die off, not everything Waked you up or hurts. Many ways of daying include lost consciousness anyway even if you're awake think of monooxide poisoning for example.

Once a friend of mine had a dream where he knew that his heart has stopped. He was feeling calm yet little edgy because he 'was aware' of the situation. He told me that he could feel/know or hear that someone "said" it wasnt his time to go yet, so he would hit himself in the chest few times and say 'lets just jumpstart the heart again' while feeling super calm or 'normal/natural', and he woke up with a pain in his chest, right above his heart where he hit himself in that dream. He was sleeping with a sport watch on his wrist, which he had forgotten to take off, he checked the info about his heartrate the same day and was little shooked, because it showed that his heartbeat had gone waayyyy too low during that night, around 30bpm.. so he didn't know about it until he "woke up" in that dream-like state.

I imagine lots of people have seen others die in their sleep. They would be able to answer?

They would be able to give some insight, absolutely. But to fully say what the experience is like, I just don't think we have the means

Really depend on what actually kill you while you're sleeping. Is it old age and your system just shut down on its own, or internal bleeding, or a stroke.... So yes to both, you can just slide off into unconsciousness and slip away and not feel anything or you can also got wake up, suffer a burst of pain then die.

It depends on what is killing you, certainly for some conditions you will never be aware of it for others you might wake up and die.

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