Sometimes rational thought seems to be bypassed in the brain for more automatic behaviour (shortcuts, instinct, intuition, heuristics):
In The Willpower Instinct by Kelly McGonigal she writes that high stress can cause automatic 'fight-or-flight' behaviour, which causes the prefrontal cortex for rational thinking to be bypassed. Relaxing yourself can help in reversing the process and becoming more rational again. She calls it the 'stop-and-think' response.
In Thinking Fast and Slow by Daniel Kahneman he writes that when someone feels confident that there are no threats facing him their system 2 (rational system) becomes lazy, 'cognitive ease' and starts to depend on heuristics too much.
So, apparently mental shortcuts happen both in situations of high stress (fight or flight) and in situations of confidence and relaxation (cognitive ease).
Question: To what extent is it indeed factually true that both too much relaxation and too much stress can cause mental shortcuts? Please elaborate...