Cortisol AM test and PM test: timing, fasting and what to expect

A cortisol AM test is a blood sample taken in the morning, when cortisol is normally at its highest. A PM sample is taken later in the day, when it should be lower. Put the two together and you can see how your level moves across the day, which one sample can’t show.

Timing is where these tests go wrong. A sample taken at the wrong hour can’t be compared with a range. This page covers when each sample is taken, whether you need to fast, how long the draw takes and what to tell the lab.

Cortisol AM and PM

A morning result and a later one, because cortisol changes through the day. Follow the times on your lab order.

$129

The seller's page for this test gives two different times for the second sample and doesn't say whether it is one visit or two. Check before you order.

See this test at HealthLabs

Price checked November 2026. Not available in ND, RI, NY, NJ, MD and HI. We earn a commission on sales.

Why the sample is taken in the morning and again later

Cortisol follows a daily rhythm. It starts to rise from about 3 a.m. in people who are awake by day, and it is highest in the morning (Diagnostics, 2023). A 2025 review describes the peak as coming 30 to 45 minutes after waking, then a slow fall through the day to the lowest point in early sleep (Int J Mol Sci, 2025).

This is why a single value is hard to read. A 2024 review in Endocrine Reviews says a value taken at a random time is difficult to interpret, because early-morning levels are generally high and levels at midnight are usually very low (Endocrine Reviews, 2024). A morning sample and a later one put two points on that curve. Our page on when cortisol is highest explains the rhythm, and the cortisol awakening response page covers the rise right after waking.

When to take the morning (AM) sample

MedlinePlus says only that the first sample is taken in the morning, when levels are normally highest. It gives no clock time. The seller’s page for its morning-and-later test says the morning collection should typically be between 7 a.m. and 9 a.m. A 2025 review that lists blood ranges uses 8 a.m. as its morning time point (Int J Mol Sci, 2025).

The pattern follows your sleep, not the clock. A laboratory review notes that the lowest point can be affected by work and sleep patterns, with shift workers as an example (Diagnostics, 2023). The seller’s page also says its timing may be reversed if your sleep schedule differs. If you work nights or keep unusual hours, ask a clinician which times suit you before you book.

When to take the later sample

MedlinePlus puts the second sample at around 4 p.m. The same 2025 review lists a 4 p.m. range alongside the 8 a.m. one. The seller’s page gives a typical window of 3 to 5 p.m. for its later sample, but another sentence says the second sample will be taken “at night”. Those are different things, so check the order before you rely on either.

A PM sample is not the late-night test. Tests for Cushing’s syndrome measure cortisol in saliva late in the evening, when levels should have dropped. NIDDK describes this as a late-evening sample you can collect at home. A review adds that samples at 10 p.m. and 11 p.m. have performed well, though midnight is the time guidelines name (Diagnostics, 2023). An afternoon blood sample does not answer that question. See saliva, urine and hair tests for how the late-night test works.

One visit or two?

MedlinePlus says a blood test of this kind needs two appointments at different times on the same day. The seller’s page for its morning-and-later test never says whether the order covers two visits, one visit or two trips to a lab. Before you pay, find out:

• Does the order cover two draws, and are both included in the price?
• Can both be done at the same lab, and at what hours?
• Is the second draw time fixed, or can you choose within a window?

If the page can’t answer, ask the seller. Don’t assume.

Do you need to fast?

MedlinePlus’s cortisol page lists no fasting rule for a cortisol blood test. Its general guide to lab-test preparation does list the cortisol test among those with special steps: rest for a while before the sample, and possibly go without eating, drinking or brushing your teeth for a set time. The cortisol page ties that last step to saliva kits. Neither of the seller’s cortisol pages states a fasting rule.

So the honest answer is: follow the instructions on your lab order, and if they are silent, ask the lab. If your clinician also orders ACTH, that is different. MedlinePlus says you may need to fast for it and that ACTH samples are often taken early in the morning. See the ACTH blood test.

How long does it take? MedlinePlus says the blood draw itself usually takes less than five minutes. How soon results arrive depends on the lab and the seller. Check the order page.

How to prepare

1. Rest first. MedlinePlus says stress and exercise can raise cortisol, so you may need to rest before the sample.
2. Tell the lab what you take. That includes steroid medicines of any kind, skin creams, birth control pills and other oral estrogen, and biotin. Estrogen raises the carrier protein that binds cortisol, so total cortisol in blood reads higher. Biotin can interfere with certain immunoassays, and some lab methods can read a steroid medicine as cortisol (Diagnostics, 2023).
3. Don’t stop any medicine to prepare. MedlinePlus says to keep taking medicines unless your provider tells you otherwise. If you take a steroid medicine, ask your prescriber how it affects the test.
4. Note the real times. Your result only makes sense next to the time each sample was taken.

What the pair can and can’t show

A morning and later pair can show whether your later value is lower than your morning one, the way the daily curve predicts. The published ranges differ by source. One 2025 review lists total serum cortisol of 3 to 23 µg/dL at 8 a.m. and 3 to 13 µg/dL at 4 p.m. Labs differ, so your report’s range comes first. See normal cortisol levels.

It can’t diagnose anything on its own:

• Cushing’s syndrome. NIDDK says no one test is perfect and doctors usually do two of a 24-hour urine test, a late-night saliva test and a low-dose dexamethasone test. A laboratory review says combining early-morning and late-night total cortisol to look for lost daily variation has been studied but is not standard practice (Diagnostics, 2023).
• Adrenal insufficiency. NIDDK says the ACTH stimulation test is the one used most often to diagnose it. A 2026 study of morning cortisol in suspected central adrenal insufficiency reports that guidelines recommend stimulation testing when a morning value falls between the clearly low and clearly sufficient cut-offs (J Endocrinol Invest, 2026).

An unusual result is a reason to talk to a clinician, who may repeat the test or add others. See testing for high cortisol and testing for low cortisol.

If you take steroids or feel very unwell

If you take a steroid medicine, never stop or change it because of a test. Ask your prescriber. NIDDK says people with adrenal insufficiency who take corticosteroid pills need immediate medical attention if they have a severe infection or diarrhea, or are vomiting and can’t keep the pills down.

If you are very unwell, with severe weakness, repeated vomiting or fainting, call 911 or go to urgent care. Don’t wait for a test. See adrenal crisis for the signs.