The parasympathetic nervous system is the division of the autonomic nervous system that calms the body and conserves energy after periods of stress or arousal. It promotes rest, digestion, and recovery.

When the parasympathetic system is active, heart rate slows, breathing becomes deeper and slower, pupils constrict, and digestive processes resume. The system acts largely through the vagus nerve and through sacral pathways that influence the lower abdomen. Its effects generally oppose those of the sympathetic division, although the two systems are continuously balanced rather than strictly alternating.

Chronic underactivity of parasympathetic tone is associated with sustained stress and certain health problems. Practices such as slow breathing, relaxation training, and some forms of meditation increase parasympathetic activity and are used to counteract sympathetic overactivation. The rest-and-digest state is as essential to long-term health as the fight-or-flight response is to immediate survival.

After the emergency passes, the parasympathetic system helps the body return to baseline.