When a person is at rest, meditating or reading a book, the parasympathetic nervous system is at work. 

Also known as the "rest and digest" system, the parasympathetic nervous system is responsible for energy conservation and maintaining vital bodily functions, including digestion, urination and defecation, especially when the body is at rest. The "rest and digest" system helps the body maintain homeostasis, or a relatively stable internal state, by balancing out the activity of the sympathetic nervous system that triggers the "fight or flight" response.

Source link