Cortisol, explained: what your stress hormone actually does

Cortisol has become the hormone people blame for everything: belly fat, 3am waking, a puffy face, feeling wired. Some of that has a basis. Most of it is a real hormone being asked to explain an ordinary life under load. Here is what it actually does.

Cortisol is a steroid hormone released by the adrenal glands on instruction from the brain. Its best-known job is the stress response, but its main job is timekeeping. Levels start rising in the early hours, jump by 38 to 75 percent in the first 30 to 45 minutes after waking (the cortisol awakening response), then decline in two phases through the day to a low point around midnight.

That morning peak is not stress. It is the body getting ready to be awake: mobilising glucose, raising blood pressure, sharpening attention. A flat morning curve is associated with worse outcomes in the research, not better. The goal is a good rhythm, not a low number.

Acute stress adds a short pulse on top of the curve. That is the system working as designed: a demand arrives, cortisol rises, the demand passes, cortisol falls.

Chronic stress does something different. The curve flattens. Evening levels stay higher than they should, the night-time low comes later, and the morning rise gets blunted. People with sleep maintenance insomnia show higher cortisol between roughly 2am and 5am than good sleepers, which is one reason stressed people wake early and cannot get back down. Why you wake up at 3am covers that pattern.

Search that phrase and you get a list: weight gain around the middle, poor sleep, anxiety, fatigue, cravings, a puffy face. Two things are true about that list.

First, most of those symptoms are what chronic stress and short sleep produce, with or without a measurably abnormal cortisol level. Stress makes people eat more in the evening, sleep worse, and feel wired; those are behavioural and sleep effects that cortisol participates in but does not cause on its own.

Second, genuinely and persistently high cortisol is a medical condition, Cushing's syndrome, and it looks different: thinning skin, easy bruising, purple stretch marks, muscle weakness, a specific pattern of fat gain. It is rare, and it is diagnosed with late-night saliva tests and suppression tests, not a symptom checklist. If you have those signs, see a doctor. If you have the first list, you are probably stressed and short of sleep, which is a more common and more fixable problem.

Saliva tests capture a moment; because the curve moves so much across the day, a single sample tells you very little. Hair cortisol averages roughly the last three months and is used in research on chronic stress, but there are no agreed cut-offs for an individual to act on. A wearable does not measure cortisol at all. Any app that says it does is estimating from heart rate.

What you can measure well are the inputs: how long you slept, when you last had caffeine, how loaded the day was, whether you moved. Those shape the curve more than anything you can buy, and a two-minute daily record captures them. How to track stress shows the method.

• Morning light, soon after waking. It anchors the curve. Ten minutes outside beats a lamp.
• A fixed wake time. The awakening response is tied to waking, so an erratic wake time gives you an erratic curve.
• Caffeine before mid-afternoon. Late caffeine keeps the evening decline from happening on time. The caffeine cut-off has the numbers.
• Some way of putting the day down. Cortisol falls in the evening when the brain judges the threat has passed. A worry window or a written list of tomorrow's loose ends helps it reach that judgement.