Cortisol awakening response (CAR): what it is, how it is measured and what blunted means

The cortisol awakening response, or CAR, is a sharp rise in cortisol in the first hour after you wake up. It sits on top of the normal daily curve. Researchers measure it in saliva, and most of what is known about it comes from research studies, not routine clinical care.

If you searched for it because of anxiety, exhaustion or a “blunted” result, this page covers what the term means, how it is measured and where the evidence stops. It doesn’t help you interpret your own saliva result. A clinician does that.

What the cortisol awakening response is

Cortisol already rises through the late night. The CAR is an extra jump on top of that. A 2022 laboratory study describes it as an increase of 50 percent or more shortly after waking from night-time sleep, with the peak 30 to 60 minutes after waking (Front Neurosci, 2022). A 2017 review gives the peak as typically 30 to 45 minutes after waking, and another 2017 paper says cortisol rises within the first 15 to 45 minutes. Sources differ a little on the exact window.

Researchers treat the CAR as distinct from the basic daily rhythm. It needs the move from sleep to waking, or the expectation of waking. A CAR can also follow daytime sleep, including naps.

Why it exists isn’t settled. One view is that it helps prepare the body for the day ahead, including changes in posture, energy demand and social contact. A 2017 paper says the function is unknown. The mechanism is not fully worked out either. For the wider pattern, see when cortisol is highest.

What affects it

Because the CAR is mainly a research measure, many of the studies are small. The main findings:

When you wake. The 2022 study used a controlled sleep-wake schedule in 34 adults. The rise was largest after waking at about 3 hours before the person’s usual waking time, and there was no rise from waking in the afternoon or evening. So the CAR depends on the body clock as well as on waking itself.
Sleep and light. Studies have linked the CAR to prior sleep and to light exposure, but results are inconsistent and the CAR also happens in dim light.
Exercise. A 2017 systematic review found mixed results. In four of five studies of aerobic exercise in non-athletes, there was no significant change in the CAR. The authors say more research is needed and that study methods and sampling differ.
Stress and mood. See below.

How the CAR is measured

The CAR is measured in saliva. A first sample is taken as soon as you wake, and at least one more follows. In the 2022 study, samples were taken before eating or brushing teeth. Most studies take the second sample about 30 minutes later. The 2022 study took it at 50 minutes. A 2017 review found no agreement on the number or timing of samples, and suggested samples at least every 15 minutes for the first hour.

Timing is the weak point. That review notes that self-reported wake times and sleep monitors don’t show when each saliva sample was actually taken, and that short delays in sampling can change the CAR. This is one reason home CAR results are hard to compare.

Saliva kits for ordinary cortisol testing, as MedlinePlus describes them, say not to eat, drink, brush or floss for 30 minutes beforehand, and to hold a swab in the mouth for about 2 minutes. Follow the instructions on any kit exactly. See saliva, urine and hair tests.

The CAR, anxiety and a “blunted” response

This is the topic most readers come for, and the honest answer is that the research is mixed.

A 2017 paper’s literature summary says an elevated CAR has been associated with major depression and possibly with anxiety disorders, and describes a large study in which adults with a depressive or anxiety disorder had an elevated CAR. The same summary says results are mixed when people are scored on traits such as neuroticism. In the paper’s own small study of 32 college students, the CAR was linked to depressive symptoms but not to anxiety or stress scores.
It also says job stress and general life stress were positively linked to the CAR, while fatigue, burnout and exhaustion were negatively linked. In other words, studies have found larger responses in some settings and smaller ones in others.
A 2017 review notes that the CAR has been related to burnout, chronic fatigue, stress, depression and post-traumatic stress disorder, and says it is unclear what counts as a “healthy” CAR, since both elevated and reduced responses have been tied to poorer psychosocial health.
The 2022 study adds that studies of the CAR in depression disagree, some finding a smaller response and some a larger one, and that this inconsistency prevents the CAR from being used as a standard diagnostic tool.

What “blunted” means, then, is a rise smaller than the comparison group’s. It describes a pattern in a research study. It is not a condition, and there is no validated cut-off that tells one person’s CAR is abnormal. A small association in a study group doesn’t tell you what a single person’s result means, and it doesn’t show that cortisol causes anxiety.

If you feel anxious or exhausted, that is worth discussing with a clinician whatever any saliva test says. A saliva CAR won’t be what they use to work it out.

Can you get your CAR tested?

A few points will keep you from wasting money.

It is not a blood-test question. A routine blood cortisol is taken at one time. It doesn’t capture the change from waking to 30 minutes later. Blood tests of the kind linked on this site don’t measure the CAR.
The evidence for commercial saliva panels is weak. A 2024 paper in the Journal of the Endocrine Society says the multi-sample saliva cortisol profiles sold for “adrenal fatigue” have never been scientifically validated. See adrenal fatigue.
Home kits have limits. MedlinePlus says at-home tests are not a replacement for guidance from a provider, and that you should follow up with your provider regardless of the result.
Tests used for adrenal conditions are different. Clinicians use blood cortisol, ACTH and stimulation tests for suspected low cortisol, and late-night saliva, 24-hour urine and a dexamethasone test for suspected high cortisol. A single measurement can’t diagnose or rule out either. See the cortisol test guide.

If you take a steroid medicine, ask your prescriber how it affects any cortisol test. Don’t change or stop it.

Questions people ask

Is the CAR the same as the morning cortisol peak?

Related, but not the same. The morning peak is the highest point of the 24-hour curve. The CAR is the extra rise after waking, which researchers treat as a distinct response layered on top of the daily rhythm. They fall at around the same time of day in someone who wakes at their usual hour, but they are measured differently, and the CAR depends on when you wake.

Does the CAR apply if I work nights?

The 2022 study found the CAR follows the body clock, and suggested it could be smaller when people wake at unusual body-clock times, such as shift workers waking in the evening. That is a laboratory finding and not a clinical test. The authors say consensus guidelines advise postponing CAR measurement after circadian disruption such as shift work or travel across time zones.

Is a blunted CAR the same as adrenal insufficiency?

No. They are different questions. Adrenal insufficiency is a medical condition, and NIDDK says it is worked up with blood tests of cortisol and ACTH, often with an ACTH stimulation test. A blunted CAR is a pattern described in research studies. See low cortisol for the condition.

Keep reading

When is cortisol highest?
The full 24-hour pattern and what shifts it.
Saliva, urine and hair tests
What each sample type can and can't show.
AM and PM timing
How blood samples are timed across the day.
Adrenal fatigue
Why it isn't a recognized diagnosis.