Calming the sympathicus is one of the very few things that somewhat works for me. It helps to relax and get good sleep.
One technique I'll add to the list: skin massage (literally pinching it with your fingers) around the sympathicus area: along the lower ribs, up the sternum and into a V-shape around the neck (roughly following the collarbones). The nerve endings on the skin literally stimulate the sympathetic system, allowing it to "reset". I don't know the exact mechanism, but I can vouch for it actually working.
I learned about it from a physical therapist. This seems to be old knowledge that was somewhat widespread (in the german-speaking world at least), then discredited in modernity as medicine was looking to be more evidence-based, and is now making a slow comeback.
NB: I have nothing against evidence-based medicine. But our bodies are much weirder than the high modernists were willing to admit.
One technique I'll add to the list: skin massage (literally pinching it with your fingers) around the sympathicus area: along the lower ribs, up the sternum and into a V-shape around the neck (roughly following the collarbones). The nerve endings on the skin literally stimulate the sympathetic system, allowing it to "reset". I don't know the exact mechanism, but I can vouch for it actually working.