Experimental Alteration of the Circadian Rhythm in Plasma Cortisol (17-OHCS) Concentration in Man

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Abstract
In an attempt to ascertain whether the pituitary-adrenal cycle of normal man necessarily has a 24-hr period or whether it has such a period merely because it is linked to a habitual sleep-wake cycle of 24-hr duration, normal subjects underwent experimental alteration of their sleep-wake schedules. Several days were allowed for the subjects to become adapted to a given sleep-wake schedule prior to sampling of blood for 17-hydroxycorticosteroid (17-OHCS) determinations. Blood specimens were obtained hourly through indwelling venous catheters in such a manner as not to disturb sleep. Alteration of the sleep-wake schedule for a single day did not alter the plasma 17-OHCS cycle. In contrast, consistent alteration of the sleep-wake schedule for at least 1 week, using sleep-wake cycles of 12, 19 or 33 hr, did result in alteration of the period of the plasma 17-OHCS cycle. On each of these schedules the visual 24-hr period for 17-OHCS was obscured and in its place a new rhythm, synchronized with the new sleep-wake schedule, was found. The dominant pattern was one in which minimal 17-OHCS values were encountered during the early hours of sleep, a rapid regular increase was observed during sleep, and maximal concentrations were found at about the time of awakening. During the period of wakefulness, the 17-OHCS fell irregularly to reach minimal values again shortly after the subjects went to sleep. Several individuals on 19- and 33- hr sleep-wake schedules appeared to exhibit two 17-OHCS cycles per sleep-wake cycle. It is concluded that the period of the pituitary-adrenal cycle is not necessarily 24 hr in length but, rather, is a function of the duration of the subject's habitual sleep-wake cycle.