Heart stopping moments before sleep.

I get it too but in my case I'm falling off a moving bus!

I get this too afaik it's perfectly normal. I also get random clangings and bangings in my head just before I go to sleep.

Weird.

I do sometimes jolt strongly just as I'm drifting off. Enough to lift me slightly off the bed.
I've never thought of it as 'heart stopping' more as Elegran says a jolt into consciousness as every thing shuts down. Annoying but harmless as far as I know.

It is probably just the sensation that everything is relaxed and your conscious mind is switching off, but if it bothers you, then go to the doctor and get yourself checked out. It is very unlikely that your heart really stops, but if you have eliminated genuine problems, then perhaps whatever in your subconscious mind is objecting to you switching off and going to sleep may be silenced.

Are you a smoker? I was once tested for a similar problem by wearing a breathing monitor for a night - the whole equipment stank of stale smoke from previous patients who presumably were heavy smokers.

Is your bedroom well ventilated? Are you too warm in bed? Changing something may make a difference. Even lifting the head of the bed a little may be enough.

I've had this sensation on and off since I was a small child.

I used to call it falling off the pavement. I only get it once, just as I fall sleep.

I have no idea what causes it but maybe other grans are more knowledgeable.