I've thought of death a few different ways.
I imagine the event itself to be rather unpleasant while conscious. What tethers us to time? How and why do we perceive the process of entropy at a consistent, linear rate? Would our conscious selves be derailed from this track by the experience of death?
I had a really bad trip once. I ceased to experience time in a linear way, and my perception of it dilated asymptotally towards infinity. To an observer the process would only have lasted for the duration of my 6 hour high, but to me it approached eternity. I was locked into a moment which played out in a trillion different ways simultaneously as it gradually deconstructed itself to nothingness. All the while my conscious view of it regressed as I myself was deconstructed, universal entropy stripping me of the self I'd cultivated throughout my lifetime. Lots of interesting shit happened in my brain and I was fucking terrified the entire time. (I wrote about the experience in detail and I'd be happy to share it privately if it piques your interest.)
I expect that to be what occurs during death. The only reason I reached an end to the experience was because the trip itself was finite, but if death is absolute and consciousness clings as desperately to existence as mine did during those few hours, eschewing its previously linear track through time, then I believe I'm absolutely within my rights to fear it.
Alternatively, I've looked at death as the ultimate motivator. The key to meeting it gracefully is living up to your potential in the time you have. You don't want the reaper to find you cowering in fear, stinking of regret for the things you wish you had done in life. Instead, if you've done all you know you were capable of, you may greet death with a serene smile.
Perhaps my consciousness clung so desperately to itself because it wasn't ready to be let go. Maybe meeting death at the end of a fulfilling life would be different: less like feeling flushed down with the sewage and more like reaching the ocean at the mouth of a great river, free to drift apart forever.
Your question about dying while asleep makes me wonder. Is that endless moment at the end of one's last breath an entity that can only manifest in wakefulness, or would it indiscriminately invade the realm of dreams? If that moment can find your mind no matter where it hides, then it probably wouldn't matter whether your eyes were open or closed when it seized you.
Anyway I'm more careful around drugs now.