Cortisol Levels by Time of Day: Normal Ranges Chart (µg/dL and nmol/L)

Cortisol follows a steep, predictable daily rhythm. In healthy adults, serum cortisol peaks around 08:30 at roughly 11.7–20.6 µg/dL (323–568 nmol/L), falls throughout the day, and bottoms out below 2 µg/dL (55 nmol/L) shortly after midnight — a more than 8-fold swing every 24 hours. A standard 8 a.m. blood draw is considered normal at about 5–25 µg/dL (138–690 nmol/L). That means a single cortisol number is meaningless without the clock time it was drawn: a value that is healthy at 8 a.m. would be strongly abnormal at midnight.

The reference profile below is anchored to the 24-hour healthy-volunteer study by Debono et al. (JCEM 2009), which measured serum cortisol around the clock, and to NIH MedlinePlus reference values for the morning draw.

The noon and 4 p.m. values are approximations read off the healthy diurnal decline — most labs only validate a morning reference interval, and intervals vary by assay and laboratory, so always compare your result against the range printed on your own report. Conversion: µg/dL × 27.59 = nmol/L.

Late-night salivary cortisol is a different scale entirely, because saliva reflects the free (unbound) fraction:

An elevated late-night salivary cortisol is one of the three first-line screening tests for cortisol excess recommended by the Endocrine Society guideline.

What does a normal daily cortisol curve look like?

Cortisol is secreted in pulses driven by the suprachiasmatic nucleus — your central circadian clock — via ACTH. The healthy pattern is: levels begin rising during the second half of the night, surge in the 30–45 minutes after you wake, peak mid-morning, then decline in a long glide through the afternoon and evening. Debono's healthy volunteers showed a quiescent phase from about 19:43 to 05:31, with the true nadir at 00:18. In practical terms, your evening cortisol should be a small fraction — often under 15% — of your morning value.

This shape matters more than any single number. Flattened curves (low morning, high evening) show up in chronic stress research and shift work, while a preserved steep slope — high morning, near-zero midnight — is the signature of a healthy hypothalamic-pituitary-adrenal (HPA) axis. Loss of the midnight nadir, specifically, is the most sensitive early sign of pathological cortisol excess, which is exactly why late-night testing exists.

What is the cortisol awakening response, and what do blunted or exaggerated responses mean?

The cortisol awakening response (CAR) is a distinct burst layered on top of the circadian rise: cortisol jumps sharply in the first 30–45 minutes after waking, typically increasing 50% or more over the waking value before declining. The expert consensus guidelines by Stalder et al. standardize how it is measured: saliva samples at 0, 30, and 45 minutes after waking, with strict timing — a 10-minute delay in the first sample can invalidate the result.

Blunted CAR

A flat or absent morning rise has been associated in research with burnout, fatigue syndromes, PTSD, and poor sleep. But interpretation is genuinely murky at the individual level: the CAR varies day to day with waking time, light exposure, and sampling accuracy, so a single flat morning proves nothing. Research protocols require multiple days of sampling for exactly this reason.

Exaggerated CAR

An oversized morning surge has been linked to ongoing work stress and anticipatory load — your system ramping up for a demanding day. Again, the evidence is at the group level; consumer "CAR tests" sold as diagnostics for adrenal status overreach what the science supports. Treat your CAR as a trend metric across many mornings, not a one-shot verdict.

What does your 8 a.m. cortisol blood test actually mean?

Labs draw cortisol at 8 a.m. because that is when the curve is at its predictable maximum, so low values are hardest to miss. A morning serum result between 5 and 25 µg/dL (138–690 nmol/L) is within the standard reference interval.

Low at 8 a.m.: a morning cortisol below 5 µg/dL (140 nmol/L) is considered suggestive of adrenal insufficiency (Addison's disease or secondary causes) by the Endocrine Society adrenal insufficiency guideline — but the diagnosis is confirmed with an ACTH stimulation test, not a single draw. Symptoms like unexplained fatigue, weight loss, salt craving, dizziness on standing, or skin darkening alongside a low morning value warrant prompt medical evaluation.

High at 8 a.m.: a high morning value alone is a weak signal — stress, poor sleep the night before, illness, estrogen therapy (which raises cortisol-binding globulin), and even the needle stick itself can elevate it. Screening for true cortisol excess (Cushing's syndrome) deliberately does not rely on morning blood; the guideline-recommended tests are late-night salivary cortisol, 24-hour urinary free cortisol, or a 1 mg dexamethasone suppression test with a cutoff of 1.8 µg/dL (50 nmol/L).

Everything here is screening context, not diagnosis. If your results sit outside the reference range — especially low morning cortisol with symptoms — see an endocrinologist rather than self-interpreting.

Saliva vs blood vs urine cortisol: which test should you use?

The practical answer: blood at 8 a.m. is the standard screen when low cortisol is the question; late-night saliva is the most convenient first test when excess is the question — validated as a screening tool since Raff et al. (1998); and multi-point salivary profiles (waking, +30 min, noon, evening) are the only consumer-accessible way to see your actual curve shape. Urine answers "how much total," never "when."

What actually shifts your cortisol curve?

• Sleep and wake timing. The curve is anchored to your circadian clock, not the wall clock. Waking earlier shifts the peak earlier; irregular wake times smear the CAR across your sampling window and make week-to-week comparisons unreliable.
• Shift work and jet lag. In a controlled lab study, Scheer et al. (PNAS 2009) showed that living 12 hours out of phase completely inverted the cortisol rhythm — high when sleeping, low when waking — alongside higher glucose and blood pressure. This is the strongest evidence that the curve follows your internal clock even when your schedule doesn't.
• Caffeine. Caffeine stimulates cortisol secretion across the day; in Lovallo et al. (2005), daily users developed partial — not complete — tolerance, with afternoon elevations persisting even in habitual consumers. A late-afternoon espresso is working against your evening decline.
• Morning light. Bright light exposure around waking amplifies the morning rise, which is one reason CAR protocols log light conditions.
• Chronic stress. Group-level data link sustained stress to flatter diurnal slopes and altered CARs, though direction and size vary between studies — this is real but noisier than supplement marketing implies.

Cortisol also raises blood glucose — one reason chronically flattened curves travel together with worsening metabolic markers like fasting insulin.

How do you track cortisol patterns without daily lab draws?

Lab cortisol panels are too expensive and slow for continuous self-tracking, but the curve's downstream signals are free: morning energy, afternoon crashes, evening wiredness, sleep latency, and night wakings all track HPA rhythm. The useful move is logging these consistently alongside the inputs that shift the curve — caffeine dose and timing, wake time, evening light, training load, alcohol.

Log stress, energy, sleep, and caffeine in BioTrakk via chat, sync your Oura or WHOOP sleep data, and the correlation engine will test — with real Pearson coefficients and p-values, not vibes — whether your 4 p.m. caffeine actually predicts worse sleep onset. If you suspect a specific lever, run it as a structured n-of-1 experiment: two weeks of baseline, then a caffeine curfew at noon, then an honest significance-tested verdict. Evening interventions with decent evidence, like glycine before bed, make good first experiments because sleep responds within days.

And if your labs are genuinely abnormal — morning cortisol under 5 µg/dL, or a late-night salivary result above your assay's cutoff — that is a doctor conversation, not a biohacking project.