The cortisol daily rhythm: when cortisol peaks, when it falls and why timing matters

Cortisol follows a daily rhythm. In people who sleep at night it starts rising from about 4 am, peaks within three hours of waking, then falls through the day to its lowest point around midnight.¹ In a healthy reference group the peak came at about 8.30 am and the low at about 12.20 am, and the morning peak was several times higher than the night-time low.² On top of that daily curve, cortisol is released in pulses every one to two hours.¹ This rhythm is why the time of a blood or saliva test matters so much, and why a hair test, which averages months, does not depend on it.

The daily cortisol curve

The most detailed healthy reference values come from 33 people whose blood cortisol was sampled around the clock.² The rise begins from about 4 am,¹ and the rest of the day looks like this:

Times and µg/dL values are from the reference study;² the nmol/L figures, the units UK laboratories use, are our conversion at 27.59 nmol/L per µg/dL.³ Within that smooth curve, cortisol actually arrives in bursts. The adrenal glands release it in pulses every one to two hours, a pattern called the ultradian rhythm, so levels rise and fall in short waves around the main daily curve.¹

The cortisol awakening response

The steepest change of the day comes just after you wake. The cortisol awakening response (CAR) is a sharp rise in cortisol over the first 30 to 45 minutes after waking.⁴ In three studies of 152 people, from children to older adults, free cortisol rose by 50 to 75 per cent within 30 minutes of waking.⁵ A review of healthy adults put the rise in saliva cortisol at 50 to 160 per cent, an average of about 9 nmol/L (range 4 to 15 nmol/L).⁶

The CAR is real but hard to measure. There are no agreed norms for absolute values: studies report 4.7 to 18.5 nmol/L on waking and 8.6 to 21.9 nmol/L half an hour later.⁶ Because the result depends on sampling exactly from the moment of waking, an international expert panel has published consensus guidelines just on getting the timing and other conditions right.⁴ It also varies from day to day. The size of the response correlated only at r = 0.39 to 0.67 between days,⁵ and the CAR on any single day is largely determined by the situation, so two to six days of sampling are needed for a reliable measure of a person's typical response.⁷

Sleep, sleep loss and cortisol

Sleep and cortisol are tightly linked. Falling asleep suppresses the HPA axis, the hormone system that controls cortisol, typically within the first 20 minutes after slow-wave (deep) sleep.¹ Sudden changes to when you sleep therefore change when cortisol rises and falls, as the next section shows.

Losing sleep shows up the next evening. After a night of partial or total sleep deprivation, evening cortisol between 6 pm and 11 pm the following day was 37 and 45 per cent higher, and the quiet evening phase started at least an hour later.⁸ In other words, a short night delays the point at which cortisol settles down for the night. See cortisol and sleep.

Shift work and jet lag

Sudden changes to your sleep-wake cycle, such as night shifts or flying across time zones, activate the HPA axis and change the usual daily pattern.¹ For testing, that creates real problems. NICE's guidance for GPs advises asking a specialist before testing a shift worker for low cortisol, because it is uncertain when to take the sample and results can be hard to interpret.⁹ Late-night saliva tests for Cushing's syndrome are not appropriate for night-shift workers or people with variable bedtimes.¹⁰¹¹

Shift work may also leave a longer-term mark. In one study, 33 shift workers had higher hair cortisol than 89 day workers (47.32 against 29.72 pg/mg), but the difference was found only among younger workers (48.53 against 26.42 pg/mg). The younger shift workers also had a higher body mass index.¹² See cortisol and shift work.

How age and sex change the rhythm

The daily curve changes as you get older. The gap between the daily high and low narrows, the early-morning rise comes earlier, the pulses get smaller and the night-time low rises.¹ In short, the curve flattens with age.

Men have higher cortisol output than premenopausal women, with a higher peak and a higher night-time low. After the menopause the difference disappears.¹ Findings on the awakening response are less settled. A 2024 review reports a larger awakening response in men,¹ while an earlier set of three studies found a stronger rise and a later peak in premenopausal women.⁵ See cortisol in men and women and cortisol and ageing.

Why the rhythm makes single tests timing-dependent

Because cortisol moves so much in a day, a single blood or saliva result only makes sense next to the time it was taken. A value taken at a random time is hard to interpret,¹ and one NHS laboratory states that a random cortisol, taken at a time other than 9 am, cannot be interpreted.¹³ That is why NICE asks for a blood sample between 8 and 9 am when checking for low cortisol, close to the daily peak,¹⁴ and why the saliva test for high cortisol is taken at bedtime or between 11 pm and midnight, when cortisol should be at its lowest.¹⁰ Normal values for each test and time are in normal cortisol levels.

Even a sample taken at exactly the right time catches one point on a moving line. In a classic study of saliva taken at 1 pm on two occasions six weeks apart, about 75 per cent of the variation reflected the situation on the day and only about 21 per cent the person's stable level.¹⁵ Researchers estimate that you need saliva samples on at least three days for a reliable average, and up to ten days to describe the slope from morning to evening.¹⁶

Why hair is not affected by the time of day

Hair works on a different timescale. It grows about 1 cm a month and takes up cortisol as it grows, so the 3 cm nearest the scalp reflect roughly the past three months.¹⁷ In experiments, hair cortisol was unrelated to the time of day the sample was taken and showed no daily variation. Neither a bout of sweating nor a cortisol surge at the time of sampling changed it.¹⁸ You can cut a sample at breakfast or at bedtime and the result reflects the same months.

That is why a hair test is the most accurate way to measure your long-term cortisol from a single sample, and why it is far more consistent from one test to the next. The same people's hair cortisol correlated at r = 0.84 from one month to the next, against 0.38 to 0.61 week to week for saliva and 0.59 to 0.68 for 24-hour urine.¹⁹

The trade-off is that hair tells you how much cortisol you have made overall, not the shape of your day. In the same study, hair cortisol was not linked with the awakening response or with the slope from morning to evening.¹⁹ If a doctor needs to know whether your cortisol falls at night as it should, a timed saliva or blood test is still the tool. See the hair cortisol test.