The autonomic nerves include the sympathetic nerve and the parasympathetic nerve, and the two nerves are unconsciously balanced. However, stress and hormonal disturbances make it difficult to adjust the balance of the autonomic nerves, resulting in physical discomfort.
To explain the sympathetic nerves in an easy-to-understand manner, imagine hunting in the primitive age. It pumps a lot of blood in preparation for hunting, which speeds up the heart and closed the bladder sphincter to prevent peeing. Also, the pupils are widened so that you can see well. The brain is excited and the activity of internal organs is suppressed.
The parasympathetic nerves work when you are resting. Because we eat, the internal organs become active, and defecation and urination become closer. The brain becomes relaxed and sleepy.
These two nerves switch well according to the situation, but if continuous or large stress is applied, it will not be able to adjust well and the sympathetic nerve will become dominant. If that happens, you may not be able to sleep at night, or your shoulders may become stiff.