When is the best time to test cortisol? Morning vs evening explained

Take a cortisol sample soon after waking, then test again before bed, and you could get two very different results.

That’s exactly what you’d expect.

Cortisol isn’t supposed to stay at one level all day. In someone who sleeps at night and wakes in the morning, it generally rises towards waking, climbs again shortly after getting up, then gradually falls. By late evening, levels should be much lower.

So there’s no universally best time to test cortisol. It depends on what you’re looking for. And without knowing when a sample was taken, a cortisol result can lose much of its meaning.

What time should cortisol be tested?

Morning testing makes sense when low cortisol is the concern because this is when levels should normally be relatively high. Late-night testing is useful for almost the opposite reason: cortisol should be close to its daily low, so an unexpectedly high result can be revealing.

This distinction shows up clearly in clinical guidance. When an endocrinologist suspects adrenal insufficiency, NICE guidance recommends measuring serum cortisol between 8 am and 9 am in people aged one year and over.

When the concern is excess cortisol, the timing changes. The Endocrine Society guideline includes late-night salivary cortisol among the initial tests used when investigating Cushing’s syndrome.

If you’re testing repeatedly to follow a pattern rather than investigating one of those conditions, consistency matters. A result taken at 8 am today isn’t directly comparable with one taken at 4 pm next week.

When is cortisol highest during the day?

Usually around waking, assuming you keep a conventional sleep schedule.

Cortisol starts to rise before you wake. Then there’s another characteristic increase after you get out of bed, known as the cortisol awakening response. A systematic review describes this response as generally peaking around 30 to 45 minutes after waking.

From there, levels tend to drift downwards through the day. They reach their lowest point around the normal sleep period.

Of course, people’s lives don’t always run from 7 am to 11 pm. Shift work, irregular sleep, and jet lag can disturb the usual cortisol circadian rhythm, which is one reason the clock alone doesn’t tell the whole story. Research into daily cortisol patterns shows how closely cortisol secretion is tied to sleep and waking.

If you’ve already had a raised result and want to understand what might sit behind it, Rezure’s guide to high cortisol levels covers some of the factors that can push cortisol upwards.

Why is a morning cortisol test used for low cortisol?

Think about the timing. If cortisol should be near the upper end of its daily pattern in the morning, a low result at that point attracts more attention than the same number taken late in the day.

That’s why an 8 am to 9 am blood sample is commonly used when adrenal insufficiency is suspected.

For adults, NICE gives clinicians several reference points for serum cortisol measured with modern immunoassays:

• Below 150 nmol/L: Adrenal insufficiency may be present, and endocrinology referral is advised.

• 150 to 300 nmol/L: The result falls into an uncertain range, so repeat testing or specialist advice may be needed.

• Above 300 nmol/L: Adrenal insufficiency is very unlikely.

There’s an important catch here. Those are serum cortisol thresholds used within a specific clinical pathway. They aren’t saliva reference ranges.

A saliva test and a morning blood test may measure the same hormone, but that doesn’t make the results interchangeable.

Nor does an afternoon blood sample simply do the same job at a more convenient hour. Oxford laboratory guidance says a random serum cortisol taken outside the appropriate morning window isn’t useful for assessing adrenal insufficiency.

Why is late-night salivary cortisol used for high cortisol?

Late at night, cortisol should be quiet.

That makes this a useful time to look for something abnormal. In Cushing’s syndrome, the normal night-time drop in cortisol can be lost, leaving levels higher than expected when they should be near their minimum.

A saliva sample works particularly well here because it can be collected at home at the relevant time without a trip to a clinic. NHS laboratory guidance commonly places late-night collection between 11 pm and midnight, although the exact timing depends on the investigation.

One sample usually isn’t the whole story. Clinical guidance commonly calls for two late-night salivary cortisol measurements, partly because cortisol varies from one day to another. Research on late-night salivary cortisol supports its use as an established screening method for cortisol excess.

Sleep itself can muddy the picture. Rezure’s article on the stress-insomnia cycle looks more closely at the relationship between cortisol, stress and disrupted sleep.

Why can a random cortisol test be hard to interpret?

Because cortisol moves. Sometimes quite a lot.

Take a sample at noon and another before bed and you’re measuring at two different points on the hormone’s normal daily curve. Add in a hard workout, an illness or a badly broken night’s sleep, and interpretation gets harder again.

Some of the things that can affect a result include:

• Sleep: Shift work, an unusually late night or a different waking time can alter the expected pattern.

• Exercise: Strenuous activity can temporarily raise cortisol.

• Stress: Acute physical or psychological stress can affect secretion.

• Illness: Being unwell can change normal cortisol levels.

• Medicines: Steroid medicines can interfere with adrenal testing.

• Collection conditions: Food, tobacco, blood in the mouth, and poor sampling technique can affect saliva results.

This is where single-number thinking goes wrong. Cortisol isn’t like body temperature, where one measurement can immediately be compared with a familiar normal value regardless of the hour.

It’s also why claims about resetting cortisol deserve some scepticism. Rezure’s piece on cortisol detoxes explains why cortisol is a normal hormone with a changing daily pattern, not a toxin that needs flushing out.

If you’re tracking cortisol, should you test at the same time each day?

As far as possible, yes.

Suppose your first sample is taken at 8 am and a later one at 5 pm. A lower second result may simply show the normal fall in cortisol across the day. It tells you very little about a change over time.

If you want repeated results to mean something, keep the sampling conditions similar. That includes the time of collection and, where relevant, the time since waking. Studies using salivary cortisol often rely on several timed samples because one isolated measurement gives a limited view of a hormone that changes throughout the day.

It’s worth recording the basics:

• The exact collection time.

• When you woke up.

• Any major change in your sleep.

• Recent strenuous exercise or illness.

• Relevant medicines.

If an endocrinologist or another healthcare professional has given you a sampling schedule, use it. Moving the test to a more convenient hour can change what the result means.

How should you prepare for a salivary cortisol test?

The instructions for the particular test come first.

For the professional-use Cortisol Rapid Saliva Test Kit for Stress Levels sold by Rezure, the person providing the sample should avoid food, drink, chewing gum and tobacco for at least 30 minutes beforehand. Blood in the mouth can also interfere with saliva sampling.

So before you collect a sample, check the requested time, stick to the preparation period and make a note of when you tested if you plan to compare results later.

Some clinical late-night protocols also ask people not to brush their teeth shortly beforehand, because bleeding from the gums can contaminate saliva. NHS guidance makes the broader point that collection instructions depend on the method being used.

And if you’re being investigated for adrenal insufficiency or Cushing’s syndrome, the test chosen by your clinician matters as much as the time on the clock. A professional rapid saliva test isn’t a substitute for the laboratory testing pathway used to diagnose these conditions.