Figure 1.

Physiological cortisol circadian rhythm. The figure shows the geometrical mean (−) ± 2 sd values (−) of serum cortisol concentration calculated from 20 min sampling over a 24-h period in 33 healthy subjects. The fitted cosinor (−) is the average of harmonic regressions that were a fit for the individual subject data. Cortisol has a distinct circadian rhythm with a peak of 15.5 μg/dl (95% reference range 11.7–20.6) occurring at 0832 h and a nadir (time of trough cortisol level) less than 2.0 μg/dl (95% reference range 1.5–2.5) at 0018 h. The mean and 95% CI are shown for the mesor (midline estimating statistic of rhythm), acrophase (time of peak using a 24 h clock with midnight taken as origin), and nadir. mcg, μg.