I read that cortisol levels elevate due to this fight or flight mechanism and normally drop down after some time, but some people’s levels stay elevated due to adrenal system problems, and this is what maybe keeps you awake in such situations.
Cortisol testing seems fairly expensive, that’s why I never got it done
On the other hand,I too couldnt sleep after a very long hard day walking mountains or riding,but have had insomnia problems for years, and am only starting to sleep properly now, due to a recent therapy