FREE‐RUNNING CIRCADIAN PLASMA CORTISOL RHYTHM IN A BLIND HUMAN SUBJECT

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Abstract
The plasma cortisol rhythm in man has been presumed to be an endogenous circadian rhythm, synchronized by some external stimulus to an exact 24 h period. Sleep/wake and social activity cycles have been considered as candidates for this synchronizer. Previous studies have suggested that the dark/light phase shifts associated with the sleep/wake cycle may be the external synchronizer, rather than the sleep/wake cycle itself. A totally blind, but otherwise normal subject was studied for a period of 50 days. Her hourly sleep/wake status and hourly integrated mean plasma cortisol concentrations were determined and the data were subjected to non-parametric mathematical analysis. The subject had a free-running rhythm in plasma cortisol with period of exactly 24 h. In addition to the dominant 24.5 h cortisol rhythm, there appeared to be a minor 24 h cortisol rhythm with a peak that coincided approximately with the time of awakening. It was not possible to determine whether this sleep/wake-related peak represented a minor component of the circadian rhythm, synchronized by some stimulus associated with sleep/wake activity, or merely an acute response to awakening itself. Nevertheless, the 2 rhythms exhibited beating behavior, resulting in maximal peak cortisol concentrations when they were in synchrony and minimal peak concentrations when they were not. Environment dark/light phase shifts are the dominant synchronizer of the circadian rhythm in plasma cortisol concentrations in man, as they are for a variety of circadian rhythms in other living things.