A simple question, but a complicated answer. I've a misfiring autonomic nervous system (ANS). The ANS comprises the SNS and PNS, as well as the enteric nervous system (ENS), which is in the gut and contains as many neurons as the spinal cord! The ANS is part of the peripheral nervous system, which includes sensory (back to brain) and motor (from brain out) systems.

Neural circuits extend to where they need to go typically by neurotransmitters, which require receptors. Agonists specific to certain receptors amp up the signal; antagonists tamp it down. There are excitatory and inhibitory neurotransmitters.

There are lots of neurotransmitters. The most important of the PNS is acetylcholine, which also is the main neurotransmitter of the SNS for the sweat glands. I'd say the most important of the SNS is norepinephrine, tho others are important, like epinephrine. These 2 substances also operate as hormones, but their importance is reversed when acting as hormones. Adenosine triphosphate is another important neurotransmitter, which works in the mitochondria in the production and storage of cellular energy. Orexin is another neurotransmitter involved in arousal, as in not sleeping during the day.

Sometimes the PNS may be overactive OR the SNS may be underactive, and sometimes the SNS may be overactive or the PNS may be underactive. And there don't seem to be good ways to measure the activity or underactivity of each of these systems, so you don't always know which is the culprit as to a specific misfiring bodily function.

Lots of pop lit online is to activate the PNS, the rest and relax response, and, by implication, to deactivate the SNS, the fight or flee response. But too inactive an SNS means you don't sweat; too much PNS, and you suffer hypotensive episodes. Too active an SNS, you don't move your bowels; too much PNS and you can have trouble with your glottal valve--ie., have a swallowing problem.

The goal is to be in a default PNS dominant setting with SNS always operating in the background and ready for extraordinary action. We all want our SNS when attacked by a lion or when under cross-examination. But we also need the SNS working in the background governing our sweating or, in my case, regrowing hair, resolving 50 yrs of Dupuytren's contracture and hemorrhoids, and regaining and promoting gum health! The PNS and SNS are complementary and work in a strange synergy.

How to promote, not a specific system, but a neurotransmitter/receptor pair within a system? Take acetylcholine. Eggs have choline, but no food has an agonist for the receptors of which I'm aware. Benadryl is an anticholinergic (of which acetylcholine is one), but don't take it if you're old like me (dementia risk). Adenosine, which is prob more important than melatonin in the sleep cycle? Avoid antagonists like caffeine or theobromine (chocolate) (these are methylxanthines); I don't know of foods to produce adenosine, tho.

I haven't touched on dopamine, serotonin, or other well-known neurotransmitters. Nor procedures like square breathing to promote the PNS or the Valsalva breathing procedure or a hand dip in ice to promote the SNS.

I don't get the science, but we laypersons can, by carefully charting our foods and procedures, find correlations with bodily functions that are controlled by the ANS and shape, tho never control, our ANS to good effect. And everything cited above (and much more) is available online; just limit your searches to reliable cites like Mayo, Johns Hopkins, Cleveland Clinic or the numerous NIH articles online, some of which are free in their entirety and some of which are synopsized.

So treatment, as well as diagnosis, even monitoring, of a misfiring ANS is no easy matter.

Peace.