Fight or Flight Response: Definition, Symptoms, and Examples

​By Sarah Sperber
​Reviewed by Tchiki Davis, MA, PhD
What is fight or flight? Read on to learn what fight or flight is, how fight or flight works in the body, and how to calm the fight or flight response.
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You may already be familiar with the fight or flight response - a simplified term for how humans and many other animals respond to threat. However, you may be less familiar with how this natural response becomes less helpful when activated too regularly. Below, we will discuss how the fight or flight response is an evolutionary adaptation that helps us deal with immediate threats, but that is not as well-suited to present-day chronic stressors.
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What Is Fight or Flight? A Definition

The fight or flight response is a “response to an acute threat to survival that is marked by physical changes, including nervous and endocrine changes, that prepare a human or an animal to react or to retreat” (Britannica, 2019). In other words, it is what our body does when encountering a threat. Below, we will discuss what sorts of threats this phenomenon is triggered by, the specific nervous and endocrine changes referred to in this definition, and how the term “fight or flight” is an oversimplification.

Why Do We Have a Fight or Flight Response?

Evolutionarily, it makes sense that we would have an effective fight or flight response. If you think back to early humans who lived outdoors in largely untouched nature, they were much more likely to encounter threats from predators. Our fight or flight response is a great adaptation for these types of threats: if a lion is going to attack you, you want your breathing and heart rate to increase so that your limbs have more oxygen and can either fight or run away as quickly and effectively as possible.

How Does Freeze Fit in With Fight or Flight?

As mentioned above, fight or flight is a simplification of how we respond to threat. The term “fight or flight” was first introduced almost a hundred years ago by Harvard physiologist Walter Cannon (1929). While the phrase is still widely accepted as an apt representation of the human stress response, some scientists have proposed further complexities to the underlying theory. For example, a team of physicians and psychologists questioned, “Does “Fight or Flight” Need Updating?” over 15 years ago (Bracha et al., 2004). In particular, these authors present the case for four responses to stress instead of two, that occur in a specific order. These options are freeze, flight, fight, and fright.

Freeze refers to the initial response to a threat such as a predator, in which an animal becomes hypervigilant and stays still. This response is adaptive because creatures that are still are less likely to be spotted by a carnivorous mammal. As Bracha et al. highlight, freeze is the equivalent to a soldier engaging in a “stop, look, and listen” response to any sign of threat.

Meanwhile, the even less commonly known fright refers to an animal’s final attempt to respond to the threat. Fright is also referred to as “tonic immobility” or, more colloquially, “playing dead,” and relates to the fact that a predator will not continue attacking an animal that is already dead.

Video: The Fight Flight Freeze Response

Fight or Flight Symptoms

The above video highlights some of the physical and cognitive symptoms of the fight or flight response:
  • Faster, shallower breathing
  • Faster heart rate
  • Need to urinate
  • Dry mouth
  • Nausea
  • “Butterflies” in the stomach
  • Cold hands
  • Sweaty palms
  • Trembling/weak legs and hands
  • Tension in thighs, neck, and shoulders
  • Focus on negative memories
  • Tunnel vision
  • Dizziness/lightheadedness

The Amygdala's Role in Fight or Flight Responses

So what is happening in the body when the fight or flight response is triggered? Fight or flight starts in the brain: when the brain receives visual or auditory signals of threat, the amygdala (the part of the brain that is generally associated with fear) sends signals to another section of the brain called the hypothalamus, which is largely responsible for regulating hormone production. The hypothalamus is one part of the hypothalamic‐pituitary‐adrenal (HPA) axis that is often thought of in relation to the stress response. We will discuss the pituitary and adrenal glands below.

Fight or Flight Hormones

In the definition at the beginning of this article, a key component of the fight or flight response is the “endocrine changes” in the body. This phrase refers to the body’s hormones, which are chemical messengers that assist in the proper functioning of various bodily functions. For example, another hormone in the endocrine system that you may already be familiar with is insulin, which is involved in the regulation of blood sugar.

The following hormones are involved in the fight or flight response:
  • Corticotropin-releasing hormone (CRH): The hypothalamus in the brain releases this hormone, which reduces appetite and increases anxiety and selective attention. It also triggers the release of ACTH from the pituitary gland.
  • Adrenocorticotropic hormone (ACTH): ACTH signals the adrenal glands at the top of the kidneys to release cortisol.
  • Cortisol: Sometimes referred to as the “stress hormone,” cortisol increases energy, alertness, and immunity - all helpful if responding to an immediate threat. You can test your cortisol levels with this at-home kit.
  • Adrenaline: The adrenal glands also release adrenaline, which increases heart and respiration rate.

What Is Happening With the Nervous System During Fight or Flight?

Some of the above hormones act as signals for the autonomic nervous system to react in certain ways. The autonomic nervous system controls involuntary bodily activities such as heart rate and breathing and is generally thought of in terms of two branches: the sympathetic and parasympathetic nervous systems.

1. Sympathetic nervous system
Usually, when we discuss the fight or flight response, we focus first on the sympathetic nervous system. One of my professors shared the mnemonic that your sympathetic nervous system has “sympathy” for you when you’re scared, increasing bodily responses that enable you to handle whatever is scaring you. For example, so that your body can fight or run from a stimulus, your sympathetic nervous system increases your heart rate and respiration and decreases digestion.

2. Parasympathetic nervous system
The parasympathetic nervous system plays a role after the perceived threat is no longer salient. Several changes take place, including slowed heart rate and respiration. Essentially, the body returns to its resting state, giving this arm of the autonomic nervous system the descriptive phrase “rest and digest.”

Fight or Flight Examples

Our fight or flight response can be triggered by any number of perceived or actual threats, both physical and psychological. Below are some examples of situations that might trigger your fight or flight response.

Physical Threats
1. Wild animals
2. Natural disasters
3. Other humans

Psychological Threats
1. Public speaking
2. Social situations
3. Phobias

Can You Have an Overactive Fight or Flight Response?

Many of the perceived threats we encounter these days are not physical but rather cognitive - there are plenty of things we worry or stress out about that do not require a physical escape or fight. However, our bodies have still evolved to react to psychological stress in this very physical way, leading to many of the hallmark symptoms of anxiety that we feel. For example, if you are about to give a speech in front of a room full of people, you may feel nervous. Your heart rate and breathing are likely increasing, and you are unlikely to want food (as your digestive system has slowed down). Your body is ready to fight or run if needed - even though this is not helpful in this situation.

You have probably noticed that different people have different stress responses. For example, if you and a friend both encounter a bear while hiking, one of you may stay calm and collected while the other begins to panic. There are many theories about why certain people have stronger or more frequent fight or flight responses than others. For example, one theory is that we are genetically predisposed to have certain stress responses. Neuroscientists have identified a particular gene, catechol-O-methyltransferase (COMT), certain variations of which are associated with a stronger startle response (Montag et al., 2008). Other researchers highlight the effect of stressful events during childhood in altering the HPA axis involved in the stress response (Gillespie et al., 2008).

How Does Fight or Flight Relate to Anxiety?

As we have seen, the fight or flight response begins with the sympathetic nervous system stimulating various bodily responses and ends with the parasympathetic nervous system bringing the body back to rest. Ideally, this process occurs only when helpful, and effectively ends with the body returning to rest. However, due to some of the aforementioned issues, this is not often the case. 

The subjective experience of the fight or flight response is one of feeling anxiety (Robinson, 1990). If your stress response is particularly strong or frequent, you are more likely to experience chronic anxiety and mood problems (Gillespie et al., 2008). At the extreme of a dysregulated fight or flight system, people can experience panic attacks - essentially fight or flight responses to no identifiable threat.
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There is a vicious cycle here since chronic anxiety and mood problems can in turn further dysregulate the fight or flight response. Doctors have also long recognized the detrimental physical health effects of chronic stress, including cardiovascular events (Curtis & O’Keefe, 2002).

6 Ways to Calm Your Fight or Flight Response

1. Deep breathing. Methods for counteracting the fight or flight response generally involve actively doing the opposite of what your sympathetic nervous system automatically triggers. For example, while the SNS increases respiratory rate and breathing becomes shallow in times of stress, researchers have found that we can actively counteract the fight or flight response by taking slow, deep abdominal breaths (Perciavalle et al., 2017).

2. Notice your patterns. It can be helpful to pay attention to when your fight or flight response is more active. For example, I have noticed that I am more likely to be on edge and jittery if I have consumed too much coffee. Noticing this pattern helped me alter my behavior so that I now limit my caffeine intake and calm my fight or flight response.

3. Acceptance. Worrying about your fight or flight response while it is happening might send more signals to the brain that you are in danger, with the result of increasing or prolonging the response. This can be seen in the case of panic attacks, where people think that their panic attack will harm them and as a result, the attack continues. Perhaps counterintuitively, accepting the sensations of the fight or flight response as normal can go a long way towards reducing them (Levitt et al., 2004).

4. Exercise. Researchers have found links between exercise and reduced anxiety (Salmon, 2001). While the reasons for this association are still being explored, one idea is that the mild stress of exercise improves resilience to stress more generally. Other theories focus on the ability of exercise to decrease sympathetic nervous system hyperactivity (Curtis &O'Keefe, 2002).

5. Cognitive-behavioral approaches. Hopefully, having read this article, you see that the fight or flight response is not always appropriate or helpful. Recognizing when your fight or flight response kicks in and reflecting on whether or not it is helpful could help reduce this response in instances where it is not helpful. For example, if you feel yourself getting extremely anxious before a date and are considering canceling, notice this fight or flight response - you are trying to “escape” a perceived “threat.” In reality, you are not in physical danger, even though this is what your body is preparing you for. Reframing how you see the situation and your bodily responses can help calm the sympathetic nervous system.

6. Speak with a professional. In addition to potential mental health issues that a professional might be able to help you with, medical issues could also be playing a role in an overactive fight or flight response. For example, a heart arrhythmia can create a sense of panic. Additionally, beta-agonist medication, often prescribed for asthma, can activate the HPA axis and incite a sense of panic. 

More Resources That Can Help You With Your Flight or Flight Response

Here are a few more articles that may help you change your thought patterns and increase acceptance.

Books Related to The Fight or Flight Response

If you’d like to keep learning more about the fight or flight response, here are a few books that you might be interested in.

Final Thoughts on Fight or Flight

Our fight or flight response is a natural reaction that has evolved to keep us safe from potential danger. Despite the clear benefits of having such a response, many of us struggle with an overactive fight or flight response that can contribute to mental and physical health problems. By understanding why you have this response and how to manage it, you can move towards greater mental and physical well-being.

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