I used to go through stages of being very scared of going to sleep.
Sometimes when I couldn't sleep it was because of a fear of the experience of the sleep state itself - an anxiety perhaps about me not being in control? I don't know, just the idea of being asleep was frightening.
The idea of sleep used to be scary for me, because it's an unknown. We could all die at any second of any day - not necessarily from an accident, could be from anything ..heart defect, sudden cerebral bleed - god knows, there are so many things that could
potentially in theory kill us. It was difficult for me because I was very anxious, and I always thought something bad would happen to me in my sleep. One of my family members who is very old now, in her 90's, is going through a similar fear right now. The only thing with her though is that it is probably fairly likely she'll die in her sleep given her age and state of poor health. She has the same fear though, and she is the only person I've ever spoken to who has had this fear. She also wants a concious death experience.
Any way I've changed my lifestyle so much recently that is far more likely I'll die in a dreadful accident than die in my sleep - so relatively speaking, it all feels very unlikely now.
I also feel less aware of myself physically since cutting down on smoking.
You're right though about it being irrational. But so many things aren't rational, and it still doesn't make us worry any less. What I can suggest is just to do things to make yourself feel as relaxed and comfortable before you go to sleep - I like to watch a film, or listen to some relaxing music. Mantras can be very helpful...
Try breathing slowly and being physically aware. When you're physically relaxing to enter a state of sleep we all have weird things that go on (like muscle twitches etc.). I think just being aware of those and thinking 'that's OK', and learning to feel comfortable with what goes on when our bodies start to 'switch off' is very good. You might even start to like it given time.
A reassuring thought for me is that sleep is not really a state of unconsciousness at all, it just feels that way sometimes when we wake up.