So, why is it that most people like to mellow out on the john? Well, the answer is psychological. You see, your nervous system is divided into two parts. these being the sympathetic and parasympathetic systems. The sympathetic nervous system may accelerate the heart rate in response to fear or exhileration. It can also be responsible for secreting adrenaline and other substances. The parasympathetic system on the other hand, will slow down the heart rate instead of speeding it up, therefore it is responsible for relaxing activities. When waste is excreted from the body, the vagus nerve is stimulated, this nerve being the big poppa of nerves that controls relaxing activities. Some people can become so relaxed in fact that they'll pass out on the toilet, since their blood pressure drops so low.