The salivary cortisol curve: what sampling across a day can and cannot capture

A hormone with a pronounced daily shape

Cortisol concentrations rise sharply in the period around waking and decline across the day toward a low point during early sleep. The rise following waking is steep enough that a difference of half an hour in sampling time changes the value substantially. This means a result reported without the exact sampling time relative to waking is close to uninterpretable.

The rhythm is generated by the body clock in conjunction with the stress axis, so both sleep timing and recent stress influence it. Describing the shape of the whole curve rather than quoting a single value is the only approach that respects how the hormone actually behaves. Even then the curve differs between weekdays and rest days for most people, so a single day of sampling captures one instance of a variable pattern.

Why saliva rather than blood

Most circulating cortisol is bound to carrier proteins and is not immediately available to act on tissue. Saliva contains predominantly the unbound fraction, which is why salivary sampling is used in research on the daily rhythm. Saliva sampling is also non-invasive and repeatable at home, which makes multiple samples across a day practical.

Contamination is a genuine issue, since food, drink, oral bleeding and certain products affect the sample. Timing accuracy is the largest source of error in practice, because participants do not always collect a sample at the moment they record having collected it. Sampling at home therefore introduces an error that no amount of laboratory precision downstream can correct for afterwards.

What the awakening response is

The sharp rise in the period immediately after waking is treated as a distinct phenomenon with its own literature. Capturing it requires several samples in the first hour after waking, timed from actual waking rather than from a clock alarm. Its magnitude varies with anticipated demands of the day, with sleep quality and with the day of the week.

Because it is so sensitive, it is a useful research measure and a difficult one to interpret for an individual on one occasion. Studies using electronic monitoring to verify sampling times have found that unverified protocols produce substantially different results.

The adrenal fatigue question

The idea that prolonged stress exhausts the adrenal glands and produces low cortisol has been examined and is not supported as a clinical entity. Systematic reviews of the concept have found the underlying evidence inconsistent and the testing approaches poorly standardised. The symptoms attributed to it are real and non-specific, which means they overlap with many conditions that can be identified.

Genuine adrenal insufficiency is a serious diagnosis made through specific dynamic testing rather than through a salivary profile. Missing that diagnosis matters, which is the main clinical objection to attributing the symptoms to an unrecognised condition instead.

Where the measurement is genuinely used

Salivary cortisol has established clinical uses, including a late-night sample in the investigation of suspected excess cortisol. Those applications involve specific sampling protocols and thresholds validated for the question being asked. Research uses it extensively to study stress and rhythm at group level, where individual variability averages out across participants.

Applying a research measure to one person, without a validated threshold, is where interpretation runs past what the method supports. Persistent fatigue has many possible causes that are identifiable through ordinary investigation, and that is a matter for a clinician.