I've noticed that a too intense, too long or too frequent workout (weights or cardio) seem to inhibate the feeling good - serotonin - endorphins effect
Endorphins actually lower cortisol levels in the body. If you are finding that the 'feel-good' effect is not there after prolonged workouts then it could be the result of low Tyrosine and Phenylalanine levels. Tyrosine is a non-essential and Phenylalanine is an essential amino-acid.
Tyrosine is used to make dopamine, norepinephrine and epinephrine which all help to elevate endorphine levels.
Phenylalanine protects the function of endorphins and can be enhanced with the use of vitamin B6.
i would imagine its for the same reason a long marathon run or any other extremely high physiogical stress on the body leaves you sucking air and feeling lousy...the results of the bodies stress, cortisol, lactic acid, depleted gycogen reserves etc must either exert a negative feedback on endorphin release as the bodies priority is rest and to replenish depleted nutrients, or it just doesnt secrete them, as its saving them for a post workout feed, at which point (after any meal from hunger) endorphins are secreted to satiate
why is your CNS fried? Not enough rest days in the week ?
Why my CNS is fried?
Because I'm an idiot and I should reduce the # sets per WO and take a week completely off more often and/or find a hot girlfriend that keeps me away from the gym