Mental burnout - or mental exhaustion is not very pleasant, when one feels completely overwhelmed, something 'snaps' and it is hard to concentrate and maintain motivation. What are the cognitive processes that occur during a burnout? What processes allow for recovery?
The sympathetic nervous system is part of the autonomic nervous system. It controls things like the fight or flight response. It excites and relaxes the body. Low adrenalin? Blame your sympathetic nervous system. Not lasting long enough under the pleasures of sex? That's your sympathetic system for you.
The parasympathetic nervous system is another part of autonomic nervous system. It is what excites you and hungry and want to have sex. It also calms you and makes you rest and enjoy being full of food.
Basically after a large amount of thinking the sympathetic and parasympathetic autonomic systems kick in and causes you to crash. This can be avoided by increasing the amount of dopamine in your system artificially with medications for ADHD like Adderall.