Preface: Signals in the Body, Not Switches

Note

Health notice: please read this first. This book reviews neuroscience, endocrinology and dynamical-systems models. It is not medical advice and makes no diagnosis. It gives no doses of any drug, hormone or supplement, does not claim that any practice treats, cures or prevents any disease, and nothing in it replaces care from a qualified professional. People with cardiovascular disease (including arrhythmias), respiratory disease, psychiatric illness (including bipolar disorder) or endocrine disease, people who are pregnant, people taking medicines that affect heart rate, blood pressure or hormones, and shift workers should consult a doctor before trying the light or breathing practices in this book. If slow breathing makes you dizzy or gives you tingling in the hands or lips, you are breathing too deeply or too fast: return to natural breathing at once. Morning light means being outdoors in daylight; never look directly at the sun. People with eye disease or light sensitivity should consult an eye doctor first.

People often talk about “dopamine”, “cortisol”, “testosterone” and “serotonin” as if they were switches in the body: dopamine is pleasure, cortisol is stress, testosterone is confidence, serotonin is calm. Set the switches right and the person improves. This book argues that the picture is wrong almost everywhere, but the questions behind it are real. Why does ordinary effort feel flat after an evening of intense stimulation? Why does a controllable stress pass and leave no trace, while long-term anxiety changes the whole system? Why does the same contest make one person stronger and leave another with cold hands?

These questions can be put in a more accurate language: that of dynamical systems. Neurotransmitters and hormones are signals inside feedback loops, with time constants, delays and thresholds. Once the shape of a loop is understood, it becomes clear why “turning a number up” is usually not the answer, and why changing the timing, rhythm and meaning of the inputs often matters more than changing the numbers themselves.

What this book is, and is not

This is a book of models and reviews. Each chapter states a piece of established neuroscience or endocrinology, then writes the mechanism as a short Python program, so that readers can change the parameters and see what happens. Unless a source is given, the numbers in the programs are illustrative: they show how a mechanism works, not measurements of anyone’s body. Applying dynamical systems, reinforcement learning or optimisation theory to human emotion and behaviour is a cross-disciplinary analogy or simplified model, and the book labels it as such every time.

Several of the claims that prompted this book turned out, on checking, to be weaker, more complicated or in need of correction: dopamine does not encode pleasure itself; chronic stress does not always show up as raised cortisol; testosterone’s effects on human behaviour are far smaller than popular accounts suggest; the theory that low serotonin causes depression lacks consistent evidence; and slow breathing amplifies the slow swings of heart rate, to which the common RMSSD measure is not very sensitive. Where this happens, the chapters say so, and the Editorial Notes list each case.

How to read the boxes

As in other 5D Library books, different kinds of statement are marked separately.

Evidence

Evidence boxes report published findings, with their sources.

Calculation

Calculation boxes give formulas and short Python programs. Each program runs as it stands in a standard Python 3 installation, with no other software, and its output is printed beneath it.

Hypothesis · not established

Hypothesis boxes hold the author’s ideas. They are labelled “not established”, and they say what result would show them to be wrong.

Source text

Source text boxes quote classical texts, with the author’s explanation.

Note

Note boxes give cautions and explain choices.

The plan of the book

Dopamine and reward prediction error. Temporal-difference learning; how an average-reward model explains the “negative error” of ordinary effort after intense stimulation; opponent-process theory.
The HPA axis and allostatic load. A model of the CRH–ACTH–cortisol feedback loop; the difference between predictable acute stress and repeated uncontrollable stress; how allostatic load is scored.
Testosterone and the challenge hypothesis. The evidence from birds to humans and how large the effects are; cortisol’s inhibition of the HPG axis; challenge and threat appraisals.
Serotonin and light. What tryptophan-depletion studies show and do not show; retinal ipRGCs and a light-phase model of the circadian clock.
The vagus nerve, heart-rate variability and breathing resonance. Why the baroreflex loop resonates at about six breaths a minute, and which measures to use.
No single optimal state. The Pareto front and rhythmic alternation.
The classics. Mencius on nourishing qi, the Suwen on tranquil emptiness and the Zhuangzi’s wooden cockerel, read beside modern research, with the limits of the analogy.
Practice and a pre-registered trial. A four-step check of personal daily rhythm, and a randomised controlled trial comparing morning daylight plus slow breathing with usual routine.

This edition

This is version 1.0.0, the first complete edition: a preface and eight chapters, in Chinese and English. Corrections, and references the author has missed, are welcome through the 5D Library website.

Disclosure

The author uses this book as the methodological basis of his community cognition courses, 5D Ascension Cards (Dimensional Shift Card Decks) and the Yafu AI knowledge base, and so has a direct commercial interest in its subject and in the result of the Chapter 8 trial. The book promotes no course, card deck or product and makes no health or therapeutic claim; the trial is designed to be run and analysed by researchers independent of the author’s business; all the code and the study protocol are published openly for anyone to check.

Fan, Y. (2026). Dynamical Systems of Neurotransmitters and Endocrinology: Dopamine Reward Prediction Error, Serotonergic Homeostasis, HPA-Axis Allostatic Load and the Challenge Hypothesis, Read Beside East Asian Classics of Mind Regulation [神经递质与内分泌的动力系统] (Version 1.0.0). 5D Library. https://doi.org/10.5281/zenodo.23738782 · Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0).