My grandma died while in a coma... Right before she died, she let out a HUGE breath of air. Apparently your lungs hold in a specified amount of air throughout your entire life. However, upon dying that last remnant of oxygen gets pushed out of the lungs... I've heard from numerous NDE reports, that at the moment that "last" breath leaves your body, it is at that moment that your soul releases from your body. (This of course depends on what type of ailment/accident is the cause for your departure.)
The amazing part about my grandmother is that at the very moment my mom told her that, "she can go whenever she feels ready..." "that we will be with her very soon..." "and that there is no need to worry, God is with you, and all you have to do is follow the light..." It was at that "very" moment my grandmother released her last breath of air.
I remember when my grandfather passed away... He had a surreal/peaceful look on his face. To me, that's a very comforting sign.
With regards to dying in one's sleep... I believe that it can be a very peaceful/easy transition into the afterlife... It would be akin to waking up from a dream in our physical reality... However, you would be waking up in the "real" reality. How much more exciting/adventurous could it possibly get? It has always amazed me that one could be having the "worst" day in their entire life... and all that it takes is 1/10th of a second for all of their worries/troubles to disappear. For instance... when I was having a very difficult/extremely stressful day a few years ago, when I felt as if "nothing" could fix my worries... I was lying on my bed and I had, instantaneously, a vision of awareness that opened up in front of me... It was a very surreal landscape of immeasurable beauty and undefinable peace. It made "everything" that I had found to be troublesome and tiring before seem unimportant and significantly insignificant. I felt like I was at my real "home" again! I honestly believe, from the deepest treanches of my heart/soul, that this is quite similar to what the dying experience as they pass on from this life to the next... It just takes "1/10th of a second for everything to become as it "should" be... ("Now, if that's not a welcoming thought... I don't know what is!?")
"May the peace go round & round!",

PUL,
Cosmic_Ambitions