Diurnal cortisol tests measure your cortisol levels at various points, from the time you wake up until the time you go to bed. They can help a healthcare professional visualize your adrenal gland function.

Cortisol is a hormone the body makes from cholesterol. It has many functions, such as regulating immune system response and maintaining sleep-wake cycles.

Cortisol is best known as the body’s stress hormone. During stressful situations, it helps your body maintain a state of heightened arousal.

A diurnal cortisol test shows your cortisol levels over the course of a day. Here’s what you need to know to take this test.

A diurnal cortisol test measures your cortisol levels several times throughout the day, typically after you wake up, before you eat lunch, before you eat dinner, and before you go to bed. By taking more than one sample, the test shows changes in your cortisol levels over time.

Diurnal cortisol tests help healthcare professionals learn more about your endocrine system, which controls hormone secretions. Doctors often use cortisol testing to diagnose problems with the adrenal gland, which produces cortisol.

It can also help diagnose pituitary gland disorders. The pituitary gland produces adrenocorticotropic hormone (ACTH), which stimulates the adrenal glands to make and secrete cortisol.

A medical professional will order a diurnal cortisol test if they need more information about your endocrine system.

Nearly anyone can take a diurnal cortisol test. Saliva and urine cortisol tests are noninvasive and have no known risks. Blood cortisol tests are minimally invasive and have some very minor risks and side effects, such as mild pain and bruising where the blood is drawn.

Your healthcare professional will give you instructions to help you prepare for a diurnal cortisol test. For accurate results, follow their instructions carefully.

It’s important to try to relax on the day of the test, as stress can raise your cortisol levels. In addition, some diurnal cortisol tests require you to avoid physical activity for one day before the test.

You might also need to avoid certain medications or skin care products, particularly those that contain steroids. But it’s important to check with your doctor first.

Finally, if you are testing saliva, you should avoid eating, drinking, and brushing your teeth for at least half an hour before the test.

Cortisol testing can be done at home but must then be sent to a lab. Most diurnal cortisol samples are collected at home using saliva. Your doctor will let you know when you should test throughout the day.

A diurnal cortisol test kit includes up to four swabs and storage tubes for your saliva samples. Follow the instructions that come with the test, which may include these general steps:

  1. Thoroughly wash and dry your hands.
  2. Remove the swab from the packaging without touching the absorbent part.
  3. Insert the swab into your mouth for 2 minutes, moving it around to make sure it’s completely immersed in your saliva.
  4. Remove the swab and insert it back into the tube. (Some tests may require you to spit into a tube instead of inserting a swab into your mouth.)
  5. Put the lid on the tube and secure it. Place it inside the packaging included with the test kit, typically a plastic bag or envelope.
  6. Follow the kit’s instructions to label your sample. Most kits require you to write the time on either the test tube itself or the plastic bag or envelope.
  7. After you’ve gathered all required samples, take or send the test kit to a doctor’s office or lab as instructed.

Diurnal cortisol test results vary a lot from one person to the next based on factors like your age, sex, and health.

Your doctor might be looking at how your cortisol levels fluctuate from morning to night, a value known as a diurnal cortisol slope.

Your doctor might notice:

  • Low cortisol (hypocortisolism): Too little cortisol is a sign of adrenal insufficiency, also known as Addison’s disease. Most of the time, it’s linked to an autoimmune disorder that causes the immune system to attack the adrenal gland.
  • High cortisol (hypercortisolism): Too much cortisol is called Cushing’s syndrome. Cushing’s syndrome can result from long-term corticosteroid use and tumors in the pituitary or adrenal gland.
  • A flat diurnal cortisol slope: Flat diurnal cortisol slopes are when there isn’t much change between your cortisol levels in the morning and at night. The authors of a 2017 review found that flatter diurnal cortisol slopes were associated with various negative health outcomes, including depression, anxiety, chronic fatigue syndrome, and obesity.

Your doctor can help you understand your diurnal cortisol test results and take the appropriate next steps.

If your results are typical, you might not have to do anything else. If your results are too low or too high, your doctor might suggest additional tests to confirm a diagnosis of an underlying condition.

If a healthcare professional suggests a diurnal cortisol test, they will tell you how to take the test. Hospitals and clinics typically offer testing on-site or provide a do-it-yourself saliva test kit.

You can buy a diurnal cortisol test kit online for $75–$100. If your doctor did not request the test, it likely won’t be covered by your insurance.

See below for important concerns about cortisol levels.

What is a typical diurnal cortisol level?

Cortisol levels usually peak in the morning, with average salivary values ranging from 10.2­–27.3 nanomoles per liter (nmol/L). Cortisol levels are at their lowest point before bedtime, with average salivary values ranging from 2.2–4.1 nmol/L.

What is the best time of day to test for cortisol levels?

Diurnal tests show changes in cortisol levels throughout the day, so you’ll have to test more than once per day. Your doctor will tell you when to test.

A diurnal cortisol test result outside of the usual values may be a sign of a hormonal imbalance that requires treatment.