A lot. If your eyes are the windows to your soul, your optic nerves are the windows to your central nervous system. Your pupils, in turn, are the windows to your autonomic nervous system.

Your pupils mirror much of the conversation taking place in your autonomic nervous system—the nerve network involved in the stress response—because they are supplied by both its sympathetic and parasympathetic branches. When you’re aroused or excited, in either a positive or negative way, your pupils dilate, and when you’re relaxed or tired, they constrict. Their subtle movements also tell us about the more intricate components of the stress network, such the locus coeruleus, a small region in the brain that plays a key role in arousal.

One example of stress potentially playing a role in eye disease is a poorly understood eye condition known as central serous chorioretinopathy (CSCR). In this condition, fluid collects under a small area (or areas) of one layer of the retina, causing a coin-shaped area of blurriness to appear in the center of the vision. We don’t know what causes CSCR, but it is often associated with an anxious, type A personality, psychological stress, sympathetic dominance, and raised cortisol levels, among other things.

When I was a junior doctor, I developed a mild stress-associated autoimmune condition, which motivated me to learn more about stress. I was intrigued to watch the condition disappear when I started taking better care of myself, both mentally and physically.

Professionally, I’ve always been interested in understanding why many patients with inflammatory conditions find their symptoms worsen with psychological stress.

The final straw that spurred me to write this book came when I moved to Hong Kong and saw many friends and colleagues suffer from exhaustion, burnout, and stress-related conditions.

One common one is the idea that adrenal fatigue causes chronic stress. A systematic review of fifty-eight studies found no evidence for adrenal fatigue being an actual medical condition. Many of the mechanisms behind chronic stress are rooted in the brain, not the body.

The adrenal glands are one link in a chain of events that begins at the level of the brain. This chain includes three nodes—the hypothalamus, the pituitary, and the adrenals—known as the HPA axis. Under a state of chronic stress, there is aberrant feedback regulation traveling across these three nodes disturbing the chain of events across the HPA axis. The adrenals are the third link in the HPA chain, so this can manifest in inappropriate cortisol release—both too much or too little cortisol have been observed in people with chronic stress.

Another misconception is that a chronically stressed person always looks acutely stressed. When we talk about stress being harmful, we’re usually referring to chronic stress, not acute stress. The brain is an intelligent and adaptable organ. If it goes through short, isolated episodes of stress that it is able to bounce back from completely, those episodes won’t result in damage. However, intense, persistent, or repeated episodes of stress that you don’t get a chance to recover from or that you respond to in an inappropriate way, can change the calibration of some of the brain and body’s baseline parameters. This causes net damage over time. Here are some examples of the ways in which chronic stress can damage the body:

• Chronic stress can change the way the brain experiences the world and responds to stressful experiences. Weak emotion regulation, seen in chronic stress, can make benign situations appear more threatening and feel more traumatic than they are. Losing the capacity to feel pleasure drains the world of color. These brain changes can increase the risk of depression, anxiety, and other mental illnesses. They may also influence pain perception and play a role in addiction and conditions such as chronic fatigue syndrome.

• Mental distress can also affect the heart. A 2004 study, known as the INTERHEART study, showed a strong link between chronic mental stress and coronary heart disease. In 1990, cardiologists identified a disorder of the heart—known as takotsubo cardiomyopathy, or “broken heart syndrome”—where the heart takes on the shape of a Japanese octopus-trapping pot, a takotsubo. The condition is triggered by severe mental distress.

• Chronic stress can also contribute to autonomic imbalance, which has wide-ranging effects on different organ systems in the body, including the digestive system. There is an emerging link between the autonomic nervous system and the immune system that points to chronic stress playing a possible role in chronic inflammation and in autoimmune diseases.

Another misconception is that every stress-reducing strategy works equally well for everyone. For example, although some studies report stress-reduction from mindfulness meditation, one large randomized controlled study on adolescents found that it actually increased anxiety (at the group level) in males. What works for one person at reducing stress may not work for another person.

Many of us are exposed to subtle chronic stressors without realizing it. Circadian disruption from not getting enough daylight or darkness, not feeling rewarded for effort at work, and chronic loneliness can all contribute to chronic stress.

Everyone’s unique circumstances will present their own specific stressors. If you’re a long-haul airline pilot, your main trigger for chronic stress may be disordered circadian rhythms. If you’re in a difficult relationship, it might be emotional exhaustion. If you’ve just signed up for a gym membership and are exercising to exhaustion each day without recovering between exercise sessions, then your new hobby may be to blame.

When you go through acute, intense psychological stress, at least seven processes can take place in your brain and body.

1. You may become temporarily inflamed.

2. You may become insulin-resistant.

3. You may feel intensely motivated.

4. Your emotional reactions may be less regulated.

5. There is a rise in synaptic plasticity in specific parts of your brain.

6. Your body clock becomes vulnerable to dysregulation.

7. And a chain of chemical messengers is released across your brain and body.

All of these processes return to normal once the stressful experience is over.

In chronic stress, these seven processes seem to go off course, to different degrees in different people. A chronically stressed person might show signs of inflammation, insulin resistance, poor motivation, irregular body clocks, inappropriate HPA axis activity, or signs of diminished prefrontal control. Not everyone will show signs of all of these, but most chronically stressed individuals will display some.

Intriguingly, there is some evidence that while chronic stress can make these processes go awry, if these processes go awry on their own, they can in turn contribute to chronic stress. For example, if your body’s circadian rhythm is irregular and you don’t release melatonin appropriately at night, this can disturb your pattern of sleep and affect your sympathetic tone at rest and your cortisol release the following morning. This has further repercussions on your emotional arousal and stress reactivity the following day. Another example regards inflammation: Inappropriate inflammation can trigger the stress response and inflammatory agents (e.g., cytokines, such as IL-6) may reach parts of the brain and influence emotion, mood, and behavior.

I advise people to focus on keeping those seven processes balanced by taking steps to improve emotion regulation, lower the risk of insulin resistance, prevent inappropriate inflammation, keep the circadian rhythm in tune, and so on, to reduce the effects of chronic stress.

It has a role in specific contexts, but it’s not the whole story. There is a theory that when we become acutely stressed, the sensations coming from inside our bodies—such as the feeling of our hearts beating rapidly (interoceptive cues)—can make us more anxious and amplify our overall stress response. Learning to associate those sensations with positive emotions can prevent this from happening. Early observations suggest that this practice of associating those feelings with positive emotions may reduce the intensity of a stress response to an acute psychological stressor. Facing a stressful situation with confidence and a sense of control helps to calm your stress response to it.

That said, chronic stress is not simply the consequence of not having the right mind-set. It can be rooted in factors that are not affected by mind-set, such as a disrupted circadian rhythm, inflammation, exertion, and so on. Even if your chronic stress is caused by too much acute emotional stress, the right mind-set alone may not be enough to buffer its load. A broad approach that includes mind-set in addition to targeting those seven processes will be more effective.

Different strategies work in different situations. Depending on whether I’ve just experienced a stressful incident or I’m having a generally stressful time, one strategy may be more beneficial than another. Here are some of the things I do.

After acute stress:

After a stressful experience, my first priority is to do something that completely absorbs my attention and prevents me from ruminating. I might play Tetris or Lumines, or something that makes me temporarily forget about what just happened. If I can’t find something absorbing to do, I focus on my breathing and try to breathe as slowly and as deeply as I can without feeling uncomfortable. This will be different for everyone, but for me, it’s about seven breaths per minute. To stop myself from ruminating while I breathe, I might simultaneously try to count every three numbers backward from two hundred. Then, if I can step away from my desk, I’ll head out for some mild- to moderate-intensity exercise for at least thirty minutes. This could be either a brisk walk or a gentle jog, ideally in an open and green space.

In the middle of a busy day:

If my day gets intense, I take a quick time-out for fifteen minutes. I put on a pair of noise-canceling headphones, close my eyes or wear an eye mask, and listen to rhythmic drumming, keeping my attention focused on the rhythm. I started doing this after reading a few studies on the calming effects of rhythmic drumming, and it works very well for me.

During a stressful week:

During a stressful week, my priorities are to manage my light/darkness exposure, exercise, and to eat well. I’ll aim to get at least three blocks of daylight exposure throughout the day, each for at least forty-five minutes: in the morning after breakfast, at lunchtime, and in the afternoon. In the evening, I’ll wear blue-light-blocking glasses; keep the lights dim, the noise down, and the excitement to a minimum; and make sure to eat early. I will also exercise at a mild intensity each day, for a longer duration, and eat more fermented foods. I’ve been practicing hot (Bikram) yoga for many years because it helps me stay calm in stressful situations. It has recently been shown to reduce stress reactivity.

There’s an interesting connection between the pupils and circadian biology. The same pathway that carries the information that makes your pupils shrink when you shine light on them is also involved in sending your brain’s “master clock” information about daylight and darkness, which influences melatonin production. The first link in this chain is a group of cells known as melanopsin-containing ganglion cells, which are most sensitive to light at a wavelength of around 479 nanometers. These are the cells we try to avoid stimulating when we wear blue-light-blocking glasses, but bright light can stimulate them, too.