Calculator

Cortisol Rhythm Estimator

See the expected 24-hour cortisol curve for your wake time, and overlay your own 4-point salivary test results.

The whole rhythm is anchored to when you wake up, not to clock time.

From a 4-point salivary panel (DUTCH, ZRT, or similar). Leave blank to just see the expected curve. Enter any points you have; flags are computed from whichever points are present.

Taken immediately on waking

The CAR peak sample

Sample time

Sample time

How this works

Cortisol follows a strong circadian rhythm anchored to wake time, not clock time. On waking, salivary cortisol rises sharply; this cortisol awakening response (CAR) peaks 30 to 45 minutes after waking, typically 50 to 75% above the waking value (Stalder 2016/2022 consensus guidelines). Levels then decline through the day, reaching a nadir around midnight to 2 a.m., before the pre-waking rise driven by ACTH pulses in the final hours of sleep restarts the cycle.

The expected curve here is parameterized from the CIRCORT reference database (Miller 2016; 104,623 salivary samples from 18,698 people across 15 field studies): a median waking value of about 11 nmol/L, a CAR peak of about 17 nmol/L at 35 minutes (a 55% rise), then a log-linear decline (rate constant ~0.167 per hour, matching how diurnal slopes are computed in the literature) to roughly 1.3 nmol/L 16 hours after waking, a nadir near 0.8 nmol/L, and an accelerating rise back to baseline before the next wake time. Because salivary cortisol is approximately log-normally distributed, the shaded band spans the median multiplied and divided by 1.9, approximating a 10th to 90th percentile range. Values entered in µg/dL are converted at 1 µg/dL = 27.59 nmol/L.

Four educational patterns are flagged. A CAR rise under 2.5 nmol/L (the consensus responder threshold) is flagged as blunted; note that sampling more than about 8 minutes late invalidates the CAR measurement. A diurnal slope flatter than -0.06 log-units per hour, roughly half the typical decline rate, is flagged because a 2017 meta-analysis of 80 studies (Adam 2017) associated flatter slopes with worse outcomes across 10 of 12 health domains, most strongly immune and inflammatory ones. Bedtime values above the reference band are flagged as elevated evening cortisol. Three or more points below the band are flagged as low overall output.

These patterns are educational, not diagnoses. "HPA axis dysregulation" patterns on salivary panels are research constructs, and "adrenal fatigue" is not a recognized diagnosis; a systematic review of 58 studies found no substantiation for it (Cadegiani & Kater 2016). True adrenal insufficiency is a medical emergency that is ruled in or out with ACTH stimulation testing under physician supervision, not with a home saliva kit. Persistently low values with symptoms such as severe fatigue, weight loss, dizziness, or salt craving warrant prompt medical evaluation.

References

Stalder T, Lupien SJ, Kudielka BM, et al. Evaluation and update of the expert consensus guidelines for the assessment of the cortisol awakening response (CAR). Psychoneuroendocrinology. 2022;146:105946.

Adam EK, Quinn ME, Tavernier R, et al. Diurnal cortisol slopes and mental and physical health outcomes: A systematic review and meta-analysis. Psychoneuroendocrinology. 2017;83:25-41.

Miller R, Stalder T, Jarczok M, et al. The CIRCORT database: Reference ranges and seasonal changes in diurnal salivary cortisol derived from a meta-dataset comprised of 15 field studies. Psychoneuroendocrinology. 2016;73:16-23.

Cadegiani FA, Kater CE. Adrenal fatigue does not exist: a systematic review. BMC Endocr Disord. 2016;16:48.