I've read fic with Steve having a fear of losing everybody he loves again. It already happened once—he woke up and the whole world and the people in it were changed. What if he had to go through that again? It led to bouts of insomnia. I don't see it in canon, but it's a plausible characterization.

I've also read MCU Tony specifically triggered by water/choking/drowning fears from the waterboarding, which makes a lot of sense.