Oh my goodness, this rings heavily with me, as I had this same problem in my life before.
I think the genesis of at least part of this fear came when my great-great-grandmother died in her sleep. (She was a century old!) My Mother was the one who told me, told me how she died, and as much as she explained to me she "went peacefully in her sleep," all it did was terrify the heck out of me that the same fate was in store for me.
Why on God's green Earth do parents say these things to their kids, anyhoo?
Now I don't know if that's even part of the reason or not, but as long as my sentient memories allow, I recall always having been a night owl. I'm not a "day" person. Even less of a "morning" person.
But for ages, I carried that fear with me that night time was equated with death and passing, and I do understand, it can be an all-encompassing panic and fear.
Logic doesn't work, I understand.
I did the same thing with the stress test, the Holter monitor, numerous EKG's, two echo-cardiograms....the cardiologists should have built a wing with my name on it for all my visits.
It sounds, however, like you have less a fear of dying, and more a concern for your wife? Is this so? (Or am I misreading?)
Have you spoken to her? I mean, directly, honestly, openly? And,...(important part coming up)....give her time and space to reply to your fears and concerns, don't do "all" the talking. A big part is LISTENING to your wife.
M.