Cortisol: The Daily Rhythm, Stress, Sleep, Food and the Tests

Cortisol is a steroid hormone made by the two adrenal glands, which sit on top of the kidneys. It helps the body respond to stress, keeps blood glucose and blood pressure up when they are needed, and holds inflammation down. It is not a toxin to be driven to zero: it rises every morning and falls every night. In a study of 33 healthy adults the peak came at about 8.30 in the morning and the low point just after midnight. This entry belongs to NNWA's handbook of hormones and explains what moves cortisol, what does not, and which questions are a doctor's.

What does cortisol do in the body?

It prepares the body to meet a demand. The pituitary gland in the brain releases a messenger called ACTH, and ACTH tells the adrenal glands how much cortisol to make. The US National Institutes of Health list four main jobs: responding to stress, reducing inflammation, regulating blood glucose and the way food is turned into energy, and maintaining blood pressure. Cortisol reaches almost every organ and tissue, which is why both too much and too little of it are serious medical conditions and not lifestyle complaints.

The link with blood glucose is the one that matters most for food. People who have had very high cortisol for a long time often have blood glucose that stays too high, together with high blood pressure, and the same can happen with long courses of steroid medicines, which act like cortisol. The handbook's entry on insulin and insulin resistance covers the hormone that brings glucose down, and how sleep and activity affect it.

What does the daily rhythm look like?

Cortisol is highest soon after you wake and lowest around the time you fall asleep. Researchers in Sheffield took blood through the day and night from 33 healthy adults to define the pattern. The figures below are that group's averages, shown to give the shape of the curve. They are not a reference range for your own report, because laboratories use different methods and units.

Two practical points follow. A single cortisol value means little without the clock time at which the sample was taken, since a morning figure can be several times an evening one in the same healthy person. And the rhythm is tied to sleep and waking, so a late or broken night shifts it, as the sleep studies below show. An expert panel on the waking rise warned that home saliva sampling is only valid when the timing is followed closely, starting from the moment of waking.

How do stress and sleep loss change cortisol?

Stress raises it for a while, and the kind of stress matters. A meta-analysis of 208 laboratory studies found that psychological stressors increased cortisol, but that the effect varied widely between tasks. The largest responses and the slowest recovery came from tasks that people could not control and in which their performance could be judged negatively by others. The authors concluded that cortisol does not respond to every kind of stressor.

Sleep loss shows up the next evening. In young men studied in a sleep laboratory, cortisol between 6 pm and 11 pm was 37 per cent higher after a night of partial sleep loss and 45 per cent higher after a night with no sleep, and the quiet phase began at least an hour late. A Lancet study that held 11 young men to four hours in bed for six nights also found raised evening cortisol, along with lower glucose tolerance.

What do caffeine and severe dieting do?

Caffeine raises cortisol, and habit blunts the effect without removing it. In a double-blind crossover trial, 48 men and 48 women took 250 mg of caffeine three times in a day. After five days without caffeine the doses raised cortisol across the whole day. After five days on 300 or 600 mg a day the morning dose no longer did so, but cortisol was still raised for some hours after the afternoon dose. The handbook's entry on caffeine covers the amounts in tea, coffee and energy drinks.

Eating too little does the same. When 121 women were randomly assigned to different regimens for three weeks, restriction to 1,200 kcal a day increased total cortisol output, and counting calories increased perceived stress. A meta-analysis of 13 studies with 357 participants found that fasting produced a very strong rise in cortisol, while very low calorie and low calorie diets did not produce a significant one. Cortisol was raised in the first period of restriction and returned to baseline after several weeks.

The low-carbohydrate finding needs its detail. The review covered diets with 35 per cent or less of energy from carbohydrate in healthy men. Resting cortisol was higher only in the first three weeks, but cortisol after exercise lasting 20 minutes or more stayed higher on the low-carbohydrate diets. The handbook's page on the low-carbohydrate diet has the wider trial evidence.

The Indian guidance and Indian evidence

No Indian body sets a requirement or a target for cortisol, and none of the sources read for this entry reports a national survey of it. What ICMR-NIN's Dietary Guidelines for Indians, 2024, do give is guidance on the habits that the studies above connect with cortisol. They call a healthy sleeping pattern of 6 to 8 hours a day necessary to avoid unhealthy eating behaviour and the weight gain that follows. They ask adults for 30 to 60 minutes of moderate aerobic activity on at least five days a week, and describe yoga as planned activity that includes breathing exercises and relaxation.

On caffeine the guidelines advise moderation so that intake stays within a tolerable limit of 300 mg a day. They put a 150 ml cup of brewed coffee at 80 to 120 mg, instant coffee at 50 to 65 mg and tea at 30 to 65 mg. On those figures three cups of brewed coffee can reach the limit, while three cups of tea come to 90 to 195 mg.

Two Indian studies measured cortisol directly. In a randomised, placebo-controlled trial, 64 adults with a history of chronic stress took an ashwagandha root extract or placebo for 60 days; serum cortisol fell by 27.9 per cent with the extract and by 7.9 per cent with placebo. At a psychiatric hospital in Bengaluru, 54 outpatients with depression chose yoga, medication or both, and cortisol fell over three months, in more of the patients who practised yoga. That study was open-label and not randomised, so it is weak evidence.

What do trials show about food, supplements and cortisol?

Less than the marketing suggests. The best-supported statement is negative: fasting and sharp calorie cuts raise cortisol for a time, so a crash diet is a poor way to lower it. No source read for this entry shows that a particular Indian food, tea or spice lowers cortisol in a way that changes health. Regular meals with enough energy, as in the calorie calculator, are the dietary step the evidence supports.

Supplements show the gap between a blood value and how a person feels. A 2025 meta-analysis of randomised trials with 488 participants found that ashwagandha lowered cortisol by 1.16 micrograms per decilitre, yet found no significant effect on perceived stress scores. The entry on ashwagandha sets out the safety questions, including who should avoid it.

Which tests measure cortisol, and who interprets them?

Cortisol can be measured in blood, urine and saliva, and doctors choose the test according to the question. The Endocrine Society's guideline on Cushing's syndrome recommends starting with one of three tests of high accuracy, and its guideline on primary adrenal insufficiency names the short corticotropin test as the reference test. The results depend on the time of the sample and can be altered by stress, exercise, pregnancy and some medicines, so reading them is the job of the doctor who ordered them, usually an endocrinologist.

A worked example: the adrenal panel

This is an illustrative case, not a real person. A 31-year-old product manager in Bengaluru sleeps about five hours on weeknights, drinks four cups of brewed filter coffee a day, and has been eating 1,200 kcal a day for a month. She is tired by 4 pm and a wellness clinic offers a four-tube saliva panel for adrenal fatigue.

Common myths and mistakes

A second mistake is to treat cortisol as the single cause of fat around the waist. Weight gain is a feature of Cushing's syndrome, but that condition is rare: the US National Institutes of Health estimate 40 to 70 people in every million for the kind that arises inside the body. For most people a growing waist reflects energy intake, activity and sleep, and the waist to hip ratio calculator is a better first check than a hormone test.

Safety: steroid medicines and when to see a doctor

The commonest cause of too much cortisol-like hormone is medicine. Glucocorticoids, the cortisol-like steroids prescribed for conditions such as asthma, rheumatoid arthritis and lupus and often injected into painful joints, are the most common cause of Cushing's syndrome when used long term at high doses. The most common cause of adrenal insufficiency overall is stopping these medicines suddenly after long use, because the adrenal glands need time to restart. Never stop or reduce a steroid on your own; the prescriber lowers it in stages.

The same risk applies to products sold for adrenal support. The Endocrine Society warns that adrenal hormone supplements taken without need can stop the adrenal glands working, and that the glands may stay inactive for months after the product is stopped, with a danger of adrenal crisis. Nothing in this entry is advice to take, change or stop any medicine or supplement.

Is high cortisol the reason for weight gain or PCOS?

Rarely. The US National Institutes of Health note that Cushing's syndrome can be mistaken for polycystic ovary syndrome or metabolic syndrome because the signs overlap, and that it affects about three times as many women as men. A doctor separates them with the tests named above, after first asking about steroid medicines. PCOS and metabolic syndrome are far more common, and NNWA's guide to the PCOS diet in India deals with food for that condition. A cortisol value from an untimed sample cannot settle the question.

Learning this with NNWA

Hormones such as cortisol are taught at NNWA, a private skill academy in Kolkata, in online classes held in English and in Hindi. The Thyroid Nutrition course and its other courses prepare people to coach healthy clients on food, sleep and activity and to refer medical cases to a doctor. None is a degree or a dietitian qualification.