A tree branch taps on your window on a dark, stormy night, or maybe you finish watching a scary movie and turn out the lights to go up the stairs to bed. You find yourself feeling uneasy and hyper alert. Your palms sweat, your stomach flutters, your eyes get wider and the hair stands up on the back of your neck.

These are common symptoms of fear. But why do they happen? And how can you override this reaction?

The mind and body respond to fear in predictable ways because of survival mechanisms inherited from our prehistoric ancestors, who regularly faced large predators that could eat them alive. When we detect a source of stress that might pose a threat, the brain activates a series of reactions that prepare us to fight for our life or run away to escape – a reaction known as fight or flight.

Fear is regulated by a part of the brain called the amygdala. When stress activates the amygdala, it temporarily overrides conscious thought so that the body can divert all its energy to facing the threat. Neurochemicals and hormones are released – including adrenaline -- causing heart rate and breathing to speed up. Your eyes open wide and pupils dilate to take in visual stimuli that might help you survive. The body redirects blood away from the digestive system to the large muscles to prepare you to run, or fight.  

The next time you’re scared, here are some common things you may notice in your body:

Rapid heartbeat
Your heart works overtime to provide as much blood as possible to fuel your muscles and breathing. The amount of blood pumped by your heart increases five times during the fear response.

The sudden release of adrenaline causes blood to rise the skin’s surface and make you sweat. This reaction is part of the role adrenaline plays in getting your body ready to either run away or fight for your life.

Goosebumps or the sensation that the hairs on our neck are standing up, are both involuntary reactions to increased adrenaline caused by fear. Known as piloerection, this reaction serves two purposes: one is for warmth and insulation in colder climates, the other is to make you look bigger (like a cat puffing its fur up to look scarier to other animals). We don't actually have enough hair on our arms and legs to suddenly make us appear larger, but it’s a response we inherited from our ancestors.

Digestive problems
Your body redirects all the blood away from the digestive system to the large muscles. People who are scared often feel the need to urinate or experience diarrhea. This is partly a reaction to the rapid changes happening internally, but is also a sign the body is expelling unnecessary waste to prepare to fight or flee.

Feeling like you’re unable to move or to move only with wobbly knees is a reaction carried down from our ancestors. It’s a sign your brain thinks it’s best to play dead to make you less interesting to a predator – usually only when the flight and fight responses aren’t working.

Humans are different from animals in that we can override responses to fear. The parasympathetic nervous system (PSNS) kicks in to counter the fight-or-flight instinct, primarily by stopping the flood of adrenaline and normalizing the heart rate.

After the initial reaction, the PSNS helps us recognize the threat is not real and calms us down.

The brain releases neurotransmitters and hormones to bring the heart rate down and slow breathing – also called rest and digest. This process produces a sense of internal cognitive relief in the body, and that feels good. This explains why we love to ride roller coasters, watch horror movies and go to haunted houses.

So, the next time you face fear, remember your ancestors. Whether it’s a scary movie or a giant predatory beast hunting you down, the reaction is the same. You can choose to fight or flight…or you could just go to bed.