Serum Cortisol Diurnal Variation
Serum cortisol is highest in the early morning hours at 8:00-9:00 AM and lowest at midnight.
Peak Cortisol Timing (Highest Levels)
Cortisol reaches its physiologic peak at 8:00-9:00 AM (0800-0900 hours), which is why this timing is recommended for diagnostic testing to capture the maximum cortisol secretion 1.
The morning reference range for adult females is 140-700 nmol/L (5.1-25.4 μg/dL) at 0900 hours 1.
Research demonstrates that cortisol concentration increases markedly following awakening, peaking approximately 30 minutes after waking, then declining throughout the remainder of the day 2.
Multiple cortisol peaks occur during the day, with significant elevations observed at approximately 7:00 AM (primary peak), 1:00 PM, and 7:00 PM (the latter two associated with meals) 3.
Nadir Cortisol Timing (Lowest Levels)
Cortisol reaches its nadir (lowest point) at midnight, representing the physiologic trough of the diurnal rhythm 1.
The midnight reference range for adult females is 80-350 nmol/L (2.9-12.7 μg/dL) 1.
Loss of the normal midnight nadir (midnight cortisol ≥50 nmol/L or ≥1.8 μg/dL) has 100% sensitivity and 60% specificity for Cushing's syndrome, making this timing diagnostically significant 1.
Clinical Significance of Diurnal Rhythm
The progressive decline occurs throughout the day from the morning peak to reach the nadir at midnight 1.
This circadian pattern is controlled by the suprachiasmatic nucleus (SCN) and is synchronized to awakening time 2.
Never interpret a single cortisol value without knowing the exact time of collection, as the same numerical value may be normal at 0900 hours but pathologically elevated at midnight 1.
Important Caveats
Shift workers and patients with disrupted circadian rhythms should not have standard cortisol timing tests, as their cortisol peaks occur at different times and will yield unreliable results 1.
Research has documented cases where the morning cortisol rise can be delayed by 7-8 hours in individuals with delayed sleep phase, leading to misinterpretation of seemingly low morning values 4.
Cortisol measured as troponin T (cTnT) follows marked diurnal variation in healthy volunteers, with morning concentrations approximately 4 ng/L (about 1/3 of the upper reference limit) higher than evening levels 5.
The amplitude, acrophase, and MESOR (midline estimating statistic of rhythm) are reliable parameters for assessing the diurnal rhythm, with intrasubject coefficients of variation of 11.2%, 7.6%, and 6.9% respectively 3.