Severe stress or trauma can trigger your fight-or-flight response. It is your evolutionary mechanism to deal with danger. It’s hugely important - it keeps you safe, and ultimately, alive. 

But, when the trauma is severe, the neurological systems designed for survival can become dysregulated. This may lead to continued fight-or-flight responses: Post Traumatic Stress Disorder (PTSD).

Think about it, when your mind and body tell you you’re not safe, you can’t relax. Your only focus is to try to feel safe. Your mind and body won’t allow you to sleep when you perceive danger. A disorder like PTSD keeps your body in a state of fight or flight: you can’t feel safe, you can’t relax, and you certainly can’t sleep.

Fundamentally, the goal is to get the body back to a state of homeostasis - feeling safe and balanced. Activating the parasympathetic nervous system (which helps you calm down) stimulates the "rest and digest" response and returns the body to homeostasis after the fight or flight response.