What Is Stress? What the Body Does Under Pressure and When It Turns Harmful

The response has two waves. The first is the sympathetic nervous system, which fires within a second of a perceived threat and orders the adrenal glands to release adrenaline: the heart pounds, blood pressure rises, the pupils widen, blood is shunted from the gut to the muscles, and the liver releases glucose. This is the fight-or-flight reaction named by the physiologist Walter Cannon in 1915, and it is what a fright feels like. The second wave is slower and hormonal. The hypothalamus at the base of the brain signals the pituitary, which signals the adrenals to release cortisol, and over minutes to hours cortisol keeps blood sugar high, damps inflammation and digestion, sharpens some memory and suppresses the immune system's slower functions. In a real emergency both waves switch off once the danger passes, and the body returns to rest.

Hans Selye, the Hungarian-Canadian physiologist who gave the word its modern meaning in the 1930s, noticed that rats subjected to any prolonged strain, cold, injury, toxins, developed the same pattern of damage, enlarged adrenals, shrunken immune tissue and stomach ulcers, and he called the pattern the general adaptation syndrome. The human version comes from pressures that have no end point: money, a bad job, caring for a sick relative, loneliness, discrimination, the noise of a city. Cortisol stays raised, and a response designed to be intense and brief becomes moderate and permanent. What chronic stress is associated with, in large studies:

• •Raised blood pressure and a higher rate of heart disease and stroke
• •Weight gain around the abdomen, since cortisol promotes fat storage there, and higher blood sugar
• •A weakened immune response, with more colds and slower wound healing, measured in caregivers and in students at exam time
• •Poor sleep, which raises cortisol further
• •Depression and anxiety, and a shrinking of the hippocampus, the brain's memory centre, which is rich in cortisol receptors
• •Digestive problems, from the gut being repeatedly told to stop

Not all of it harms. Short, controllable challenges, a race, a performance, a hard problem, produce the same hormones and leave the body better prepared for the next one, and people deprived of all demand do worse, not better; the relationship between pressure and performance is an inverted U, rising through moderate stress and falling only when it becomes excessive. What matters is control and duration. Experiments on animals and observations of people find that the same stressor is far more damaging when it cannot be predicted or escaped, and Michael Marmot's studies of British civil servants found that those in the lowest grades, with the least control over their work, had heart disease rates several times those at the top, though the top had more responsibility. The stress that kills is the kind one can do nothing about.

The body responds not to the event but to the appraisal of it, which is why the same situation floors one person and exhilarates another. Studies in which people are told that the racing heart before a speech is the body preparing to perform, rather than a sign of anxiety, find that their blood vessels stay relaxed rather than constricting and they perform better, and a large American survey found that people who reported high stress and believed it harmful had a higher death rate over the following years, while those with equally high stress who did not believe it harmful did not. The finding does not mean stress is imaginary. It means that the interpretation is part of the physiology.

The remedies with the best evidence are unglamorous. Exercise burns off the mobilised fuel and lowers resting cortisol; sleep is both a casualty of stress and the most effective repair; and social contact is the most consistent buffer of all, with people who have close relationships showing smaller hormonal responses to the same pressures and recovering faster, which is why loneliness counts as a stressor in its own right. Slow breathing engages the parasympathetic system that opposes the alarm, and meditation and cognitive behavioural therapy both reduce cortisol and symptoms in trials. Beyond the individual, the lever with the largest effect is control: giving people more say over their work and lives reduces the harm more than teaching them to relax about its absence, which is a point for employers and governments rather than for the stressed.

Stress affects memory in two opposite ways, and the difference explains a familiar experience. During and just after a stressful event, stress hormones help the brain store emotional memories strongly, working through the amygdala, which is why you can remember exactly where you were when you heard shocking news. But the same surge interferes with the prefrontal cortex, the part of the brain that holds information in mind and searches for stored facts. So stress can make a new memory stick while making it harder to retrieve something you learned calmly weeks ago.

That is what happens when you sit down to an exam you prepared for and cannot recall a fact you knew yesterday. The knowledge is still there; the search process has been disrupted. Two things help. Practising retrieval, by testing yourself rather than rereading notes, builds stronger routes to the memory, and research suggests that well-practised memories are less affected by stress. Rehearsing under test-like conditions, with a timer and no notes, also makes the real situation feel less threatening. If your mind blanks, moving on and coming back often works, because the first surge of alarm settles within minutes.

To study stress, scientists need a reliable way to cause it in a laboratory, and the best known is the Trier Social Stress Test, developed by researchers in Germany in the early 1990s. A volunteer prepares a short speech, often a pitch for an imaginary job, and delivers it to a panel of judges who stay expressionless and give no encouragement. Then, without warning, they are asked to count backwards aloud from a four-digit number in steps of thirteen, starting again whenever they slip. Being judged without any control over the outcome raises cortisol in most people, peaking roughly twenty to thirty minutes later.

Cortisol itself is usually measured in saliva, which is painless and can be collected at home, but timing matters because the hormone follows a strong daily rhythm. It rises sharply in the first half hour after waking and falls through the day to a low point around midnight, so a single sample means little unless the time is recorded. For longer periods, researchers analyse hair. Scalp hair grows roughly a centimetre a month and takes up cortisol as it grows, so a three-centimetre snippet nearest the scalp gives an average for the last three months, a measure of chronic stress that no single test can provide.

One of the clearest demonstrations that psychological stress affects the body came from a study published in 1995 by Janice Kiecolt-Glaser and colleagues. They made a small, standard wound on the arms of women who were caring for a relative with dementia, a role known to involve long-term stress, and on a comparison group of similar women without that burden.

The carers' wounds took noticeably longer to heal, on average about nine days more. Their immune cells also produced lower amounts of a signalling chemical important for healing. Later studies found similar effects in students wounded during exam periods compared with holidays. The research helped establish that chronic stress can slow the body's repair systems, one of several routes by which long-term pressure can affect health.

Stress is the body's alarm response to a demand: a fast wave of adrenaline that readies muscles and heart, and a slower wave of cortisol that keeps fuel flowing and suppresses repair, both built to switch off when a short emergency ends. When the pressure is chronic and uncontrollable the response stays on and is linked to heart disease, weakened immunity, poor sleep and depression; short, controllable challenges do the opposite, appraisal shapes the physiology, and exercise, sleep, social contact and control over one's situation are the reliable protections.