Cortisol is a steroid hormone, produced in the adrenal glands, that regulates a wide range of processes throughout the body, including metabolism and the immune response.

It's known as the stress hormone, and works with certain parts of the brain to control mood, motivation, and fear.

It’s best known for helping fuel your body’s “fight-or-flight” instinct in a crisis, but cortisol plays an important role in a number of things your body does. For example, it:

• Manages how your body uses carbohydrates, fats, and proteins
• Keeps inflammation down
• Regulates your blood pressure
• Increases your blood sugar (glucose)
• Controls your sleep/wake cycle
• Boosts energy so you can handle stress and restores balance afterward
• Cortisol is essential for dealing with crisis situations, however

How cortisol affects thinking

Have you ever wondered why, when you're in a highly stressed emotional state, you can't seem to think?

This problem is the foundation of such human challenges as;

• stage fright
• social anxiety
• approach anxiety
• test anxiety
• fear of confrontations
• some forms or sexual or erectile dysfunction

Cortisol's job, in all mammals, is to activate your threat response system- meaning to push you to fight, flight or freeze, in response to a perceived danger. 

But as the human brain evolved from mammals, the addition of the pre-frontal cortex could not impede those survival mandates - so evolution designed a mechanism for that. When Cortisol is present at high levels, your pre-frontal cortex effectively shuts down.

Why? Because if a lion steps out from behind a tree, you don't need the pre-frontal cortex to say "wow, what a beautiful mane, that is one stunning creature". Or, "gee, lions are pretty fantastic killing machines, I wonder how they evolved... let's see..."

Instead, you need to freak out, and probably climb the nearest tree as high as you can. ( Note this is not a recommendation, I have no idea how to escape a lion ).

Understanding the connection between stress, and your ability to think, is important. Learn to react to the stress directly, and deal with it first.

Learn more...

• Neural circuits responsible for conscious self-control are highly vulnerable to even mild stress. When they shut down, primal impulses go unchecked and mental paralysis sets in
• Stress signalling pathways that impair prefrontal cortex structure and function

Chronic stress

If you’re under constant stress and the alarm button stays on, persistently high cortisol levels can derail your body’s most important functions. It can also lead to a number of health problems, including:

• Anxiety and depression
• Headaches
• Heart disease
• Memory and concentration problems
• Problems with digestion
• Trouble sleeping
• Weight gain

Learn some tips on chronic stress, and how to deal with it, at the Mayo clinic.