The sympathetic nervous system is short handed as Fight or Flight
The PARAsympathetic nervous system is Rest & Digest.
When one system turns on? The other turns off. It’s not quite binary, but it’s close, more like a dimmer switch. The more one system is on, the less the other is. It’s because we have finite energy resources.
A low level “on” of the sympathetic nervous system? Will cause lack of appetite as your digestive system starts to shut off, longer healing time / ease of infection, lack of sleep or disturbed sleep, feeling cold (as your blood moves from your limbs to your core), etc.
A high level “on” of the sympathetic nervous system will cause voiding of the digestive system (puking, pissing, and shitting yourself), your immune system to shut off entirely, being wide awake for days at a time, and no sensation of pain. Amongst other things. Like the ability to lift a car off of someone and to hold your breath for several minutes.