Twenty-four Hour Pattern of the Episodic Secretion of Cortisol in Normal Subjects

Plasma cortisol was measured for seven 24-hr periods using the frequent sampling technique (every 20 min) in 6 normal acclimated subjects. A defined sleep-wake schedule was established over 3–5 consecutive nights with polygraphic definition of their sleep patterns. A mean of 9 secretory episodes (range 7–13) occurred over the 24-hr sampling time, the subjects spending an average of 24% of the time in active secretion. It was estimated that, on the average, 16 mg of cortisol was secreted over the 24 hr, with a mean of 66 min half-life of cortisol decay. Although great variability was found in both the amount of cortisol secreted and the time spent in secretory activity/hr, the secretory rate was quite constant at approximately .05 mg/min. A temporal pattern of episodic secretion was recognized, and the 24-hr sleep-wake cycle could be divided into 4 unequal temporal phases: Phase 1. A 6-hr period of “minimal secretory activity” (4 hr before and 2 hr after lights out); Phase 2. A 3-hr period called “preliminary nocturnal secretory episode” (3rd to 5th hr of sleep); Phase 3. A 4-hr period, the “main secretory phase” (6, 7, 8 hr of sleep and 1st hr after awakening); and Phase 4. The 11 hr of “intermittent waking secretory activity.” No evidence for a “basal level” or “steady state” of cortisol concentration was found. Changes in cortisol output during the 24-hr day appear to be due to differences in frequency and duration of secretory episodes and not to major changes in secretory rate.