system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why people feel tired, exhausted after expressing strong emotions (breakdowns)? What is physiology behind this?
gpt
Because they *are* tired and exhausted. An episode like an anxiety attack or crying episode is extremely taxing on the brain and the rest of the nervous system. 

Let's use the example of an anxiety attack. During an anxiety attack or panic attack, the body is experiencing a sudden spike in stress, leading to a surge in adrenaline, increased heart rate, increased breathing rate, sweating, etc. 

Sounds a lot like what happens during intense physical exercise, right?

Coming down from a panic attack is usually going to involve all of those things slowly going back to normal thanks to the parasympathetic nervous system finally being able to catch up, which usually leaves a person with the feelings you stated in your question.