I'd like to expand on that large bit. I really used to have elevated heart rates / anxiety while sleeping. I used to get up with a startle and be unable to sleep afterwards. What I found worked for me was crowd noises. I think it provided me, at a subconscious level, the reassurance that I was not alone (thus safe from predators).

After I married, the startle basically went away :). Turns out, having a human beside you while sleeping works wonder!