Humans brain’s evolution, has make that even if humans are not aware, it take some actions according to the situation the whole body and mind is living, so, in a stressing situation the center to “ fight or flight” is activated. This center, called amygdala, is associated with the fear and any emotion involved from the beginning of the process of response of the body to the stress.

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The amygdala (/əˈmɪɡdələ/; plural: amygdalae; also corpus amygdaloideum; Latin from Greek, ἀμυγδαλή, amygdalē, 'Almond', 'tonsil'[1]) is one of two cherry d-shaped groups of nuclei located deep and medially within the temporal lobes of the brain in complex vertebrates, including humans
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The amygdala, also called as the sixth sense in humans, has multiple roles in our body going from:

The number of friends, social interaction, according to experts, a person with a big and well-connected amygdala has more connections, also they think that a big amygdala indicates the level of emotional intelligence.

Sexual orientation, the amygdala present specific connections patterns according to sex, so in homosexual specimen their patterns are closer to the other sex than the own.

Aggression and fear, lesions in the amygdala can change normal behavior, for instance, a person whit no fear is possible thanks to this lesion.

Anxiety, and its direct association with fear, this could be trigger in real life or in a simulate situation with the same result, high level of sugar in the blood in order to fight or run from the trigger.

Post traumatic disorder, bipolar disorder, all this are connected with a problem in the normal development in the amygdala’s functions, among others.

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Full attention to the present moment is the perfect solution if you suffer of anxiety and stress. Mindfulness

Jon Kabat Zinn defines mindfulness as:
“Paying attention;
On purpose,
in the present moment, and
non-judgmentally.”
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It’s important to say that, practice mindfulness isn’t try to control your thoughts, is just “see it” without judge anything, only paying attention to the experience.

When people decide to put full attention in what they’re doing in that moment and totally disconnect their thoughts from the past and the future, centering only in “here and now”, appreciating everything, every act, every word, every action, going with the flow, making connections with people and their conversations, all of it helps to make that the connection between the amygdala and the rest of the brain become weaker, while the connections between the areas associated with attention and concentration become stronger.

An excellent practice that can help people to reach the mindfulness awareness is initiate in the practice of meditation, there’s a lot of courses, videos and pages showing how to meditate, I even wrote a post about this sometime ago, you can read it here

Is also known thanks to magnetic resonance images that after a few hours of practice mindfulness, the amygdala’s size is smaller, as shrunken. With all the consequences of that reaction, here we must concentrate in the relief to feel calm and aware at the same time, without trace of anxiety, angry, depression and sadness. Besides, the pre- frontal cortex seems to grow and get thicker, which make us feel more compassion, kindness, empathy and a higher level of reasoning skills.

Mindfulness is the core essence of every wisdom tradition through history

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Summary

Mindfulness make, like games and puzzles, that our mind automatic disconnect from its stress center, yoga and meditation are some ways to do it, but there’re countless ways to do it. Search and apply the one that fit your life, you’ll feel and enjoy the multiples benefits that it will bring to your life, just to name a few I can mentioned:

Reduce and alleviate stress.
Higher brain activity
Lowered heart rate
Decreased depressive symptoms
Improve general health.

Among others.

So let's stay here and now, present moment and breath...

Sources:
Wikipedia
Mindfulness
Youtube
Positive Psychology Program