how can people improve their parasympathetic nervous system to improve the quality of sleep?

Answer 1

Your question presupposes that sleep quality is damaged by some type of failure of the parasympathetic autonomic nervous system. This is not the case.

In fact, there can be many reasons that people have difficulty sleeping, and by far the most prevelent involve the complexity of the human psyche. There are several medical diagnoses which can involve sleep disturbance, of course, and any responsible physician will rule out things like hyperthyroidism or other hormonal disorders, or even obstructive sleep apnea in the appropriate patients, prior to recommending psychological evaluation.

The parasympathetic nervous system is part of the autonomic nervous system. This is a series of nerves that connect from the brain to the tissues of the body and regulate organ function. Unlike the portions of the brain that we are more intimately aware of - those parts that are involved with thinking, feeling, sensing, and moving - the autonomics function independently from our awareness. These nerves modify the function of systems such as the cardiovascular system, the digestive system, the kidneys and the adrenals as well as others. The modifications of function are complex, depending on the ever-shifting balance between sypathetic and parasympathetic autonomic output to the organs and precisely what those inputs cause each organ to do differently.

Sleep is not a function of peripheral nerves. Sleep is, instead, a function of the central nervous system. No "improvement" to the peripherally disposed parasympathetic fibers would have a central effect, anyway.

If you are having longstanding trouble with sleep, then I recommend you be seen by a qualified physician.

Good luck.

...I hope that helps.