“Brain releases cortisol that raises your heart rate, it modulates adrenaline levels and it clouds your thinking.”
And with all the cortisol in your brain your thinking will be cloudy and you won't even know about it because "your thinking is cloudy".

“It's something that he had gotten from the psychologist Gary Klein, who had written about it a few years before, also called the pre-mortem. Now, you all know what the postmortem is. Whenever there's a disaster, a team of experts come in and they try to figure out what went wrong, right? Well, in the pre-mortem, Danny explained, you look ahead and you try to figure out all the things that could go wrong,and then you try to figure out what you can do to prevent those things from happening, or to minimize the damage.
said Daniel Levitin in his recent Sep 2015 TED talk, “How to stay calm when you know you’ll be stressed?”.
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