Fight or Flight: How the Sympathetic Nervous System Activates Your Body During Stress

Erwin van den Burg
23 juni 2024

What Is the Fight or Flight Response?

The fight or flight response is your body’s built-in survival system — driven by the sympathetic nervous system. It kicks in when you face a perceived threat and prepares you to take immediate action: to fight the danger or flee from it. This response is rapid, automatic, and vital for survival. But when it’s triggered too often or for too long, it can wear your body down.

How the Sympathetic Nervous System Triggers the Fight or Flight Response

The sympathetic nervous system (SNS) is part of the autonomic nervous system, which controls functions you don’t consciously direct, like your heartbeat and breathing. When stress hits — whether it’s a traffic jam or a real danger — your SNS activates within milliseconds. This triggers:

  • Rapid heartbeat
  • Faster breathing
  • Muscle tension
  • Increased alertness
  • Sweating and dilated pupils

These reactions are designed to help you react quickly and effectively.

The Role of the Amygdala: Your Brain’s Alarm Center

The amygdala is the part of the brain that processes danger. It receives sensory information and immediately evaluates its emotional significance. If it detects a threat, it sends a signal to the hypothalamus, which activates the sympathetic nervous system.

Within seconds:

  • CRF (Corticotropin-Releasing Factor) is released in the brainstem
  • The locus coeruleus produces noradrenaline (norepinephrine)
  • Signals are sent to the adrenal glands, which release adrenaline (epinephrine) into the bloodstream

Adrenaline boosts your body’s physical readiness to handle danger.

What Happens to Your Body During Fight or Flight

Adrenaline and noradrenaline lead to:

  • Increased heart rate and blood pressure
  • Rapid breathing and oxygen intake
  • Tightened muscles
  • Heightened senses (sight, hearing, smell)
  • Glucose and fat release from storage for quick energy

These changes prime the body for immediate action — ideal if you're facing danger, but less useful when the “threat” is an inbox full of emails.

Chronic Stress and Sympathetic Overdrive

The fight or flight response is incredibly effective in short bursts. But in today’s world, stress isn’t always short-lived. Deadlines, financial pressure, and relationship conflicts can keep your sympathetic nervous system constantly switched on.

Over time, this can lead to:

  • High blood pressure
  • Heart problems
  • Weight gain from emotional eating
  • Sleep issues and insomnia
  • Mental health challenges, including anxiety and depression

How to Know If Your Sympathetic Nervous System Is Overactive

Common signs include:

  • Pounding heart
  • Sweating
  • Muscle tightness
  • Digestive issues
  • Restlessness or irritability
  • Trouble sleeping

These are clear indicators that your fight or flight system is over-engaged — even when no real danger is present.

How Stress Responses Vary Between People

Not everyone reacts to stress the same way. Studies show differences in sympathetic nervous system activation based on genetics, childhood experiences, and environment:

  • Some people show mild activation — enough to cope with stress without overreacting
  • Others show chronic anticipation, activating stress systems before a threat even appears
  • Children from high-stress or low socioeconomic households may show abnormal activation patterns

Early life stress can shape how your stress systems function in adulthood.

Measuring Fight or Flight Activation

Your SNS activity can be measured using techniques like:

  • Pre-ejection period (PEP): the time it takes the heart to push out blood (shorter = higher SNS activity)
  • Respiratory sinus arrhythmia (RSA): variability in heart rate tied to breathing, which reflects parasympathetic activity

Though not routinely used in clinics, these methods are used in research to understand stress responses more precisely.

Understanding Your Own Response Style

Even without lab tests, you can observe how your body reacts to stress. Ask yourself:

  • Do I take action when I’m stressed?
  • Or do I shut down and wait it out?

Neither is inherently better — active coping is useful when the situation can change, while passive coping may be better when it can’t. The key is recognizing your pattern and adapting accordingly.

Harnessing the Fight or Flight System

Your fight or flight response is a brilliant, evolutionary ancient tool that helps you survive and adapt. But in a world full of non-life-threatening stressors, it can become overactive, contributing to physical and mental health problems.

By recognizing the signs and understanding how your sympathetic nervous system works, you can:

  • Become more aware of your stress triggers
  • Choose the most effective coping strategy
  • Reduce the toll of chronic stress on your health

For a deeper dive into managing stress and regaining control, check out our course Surmounting Stress — where science meets practical solutions.