the lower brain is sometimes called the lizard brain, reptilian brain, or primitive brain because it controls instinctual survival behaviors that are less sophisticated. the limbic system and amygdala control primitive responses to stress / fear. these sections of the lower brain are concerned with very short-term outcomes because, for the vast majority of human existence, threats required an immediate and forceful response (fight or run). this response isn't necessary as often these days, but the software is running in overdrive by responding to perceived threats like social anxiety, financial stress, relationship drama, work pressures, overtraining, etc.

upper sections of the brain, which evolved later, control complex problem-solving, introspective thinking, emotions, language, learning, memory, empathy, and intrapersonal skills. this is where intentional thought, reason, and informed action happen. this is where conscious decisions are made. the upper brain thinks before doing and considers long-term outcomes.

the mind and body instinctively prioritize short-term threats over long-term priorities and health. when the mind-body is stressed, the lower brain is prioritized to respond to a perceived threat (even if there is no real threat). since stress response is designed for immediate action, connection to the upper brain is choked which weakens emotional control, language, problem-solving, perspective, and most processes that would help to understand a situation. your lower brain doesn't care about understanding a situation, it just needs to do something about it now. this is fight or flight.

this is why perspective is difficult when you're dealing with a stressful situation. your stressed brain wants to react and get through whatever is happening. that's been its' MO for most of human existence. it's difficult to weigh the pros and cons, to consider multiple angles of a problem, or to think objectively when the pressure is on because the connection to the upper brain is choked by fight or flight.

as stress accumulates over time, nervous system tension creeps up. as this tension rises, your bandwidth or adaptive capacity declines. if nothing is done to bring that tension consistently back to balance, your daily operating tension will rise to become sympathetic dominant on a regular basis.

when you're really tired, you probably notice you have less patience or less capacity to process new information, think objectively, and act intentionally. you may be a little more "snippy" or irritable. this is a consequence of the lower brain being prioritized when energy is in low supply. this makes a lot of sense for survival (which the stress response is designed for). if your energy and resources are low, your mind-body will err on the side of caution and crank up the sensitivity of the stress response just in case a threat is around the corner. your mind body is pulling resources away from the area of your brain associated with critical thinking and away from the adaptive immune system because these things aren't important to your immediate survival.

some people find sleep deprivation to be a good "hack" when getting the thinking brain out of the way is helpful. if the thinking brain is overthinking and paralyzing progress on a project, some people skip sleep to choke the connection between thinking and reaction to help them make a move. i've seen this most with writers.

nothing is free. the upper brain is brilliant, but it can overthink. it can ruminate and worry. the upper brain can think of a stress response into action, giving the driver seat to lower brain thoughts and responses, landing on pathways of anxiety and fear. things can spiral as the lower brain takes the wheel. ego gets involved, thoughts and actions become defensive, negative outcomes replay in your mind, and intrusive thoughts interfere with productivity and presence.

even when facing a truly dangerous situation today, you want upper brain control over reactive processes. there's very little margin for error in a life-threatening situation, and panic often results in failure. responses to threats are best when quickly informed by the upper brain. survival training is centered around keeping enough composure (front brain control) in a life-threatening situation so you can survive.