The sympathetic nervous system (SNS) prepares the body for ‘emergency’ situations. It’s for instant physical activity that requires max. energy the body can provide. It’s usually referred to as the ‘fight or flight’ response, as it prepares the body to fight the threat or run away from it. It results in things like the pupils widening to take in more light, increase of blood flow to vital parts of the body etc. Most importantly, it releases adrenaline. The parasympathetic nervous system is almost exact opposite, calming the body down after you’re clear of threat and safe, simply turning everything back to level! Note: it kicks in with acute stressors rather than chronic! Hope that was helpful!